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JC2 8986 






Bureau of Mines Information Circular/1984 



Noise Control 

Proceedings: Bureau of Mines Technology Transfer 
Seminars, Pittsburgh, PA, July 24, 1984, 
and Denver, CO, July 26, 1984 



Compiled by Staff, Bureau of Mines 




UNITED STATES DEPARTMENT OF THE INTERIOR 






Information Circular 8986 

VI 



Noise Control 



Proceedings: Bureau of Mines Technology Transfer 
Seminars, Pittsburgh, PA, July 24, 1984, 
and Denver, CO, July 26, 1984 



Compiled by Staff, Bureau of Mines 



- 



UNITED STATES DEPARTMENT OF THE INTERIOR 
William P. Clark, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 




Library of Congress Cataloging in Publication Data: 



14 



Bureau of Mines Technology Transfer Seminars (1984 : 
Pittsburgh, PA, and Denver, CO) 

Noise control. 

(Information circular / United States Department of the Interior, Bu- 
reau of Mines ; 8986) 

Includes bibliographies. 

Supt. of Docs, no.: I 28.27:8986. 

1. Noise control— Congresses. 2. Mine safety— Congresses. I. 
United States. Bureau of Mines. II. Title. III. Series: Information 
Circular (United States. Bureau of Mines) ; 8986. 

-W295. TJT" 622s [6 22'. 4] 84-600194 



CONTENTS 

Page 

Abstract , 1 

Introduction, by John N. Murphy 2 

Bureau of Mines Occupational Noise Control Program, by J. Harrison Daniel 14 

The Noise Exposures of Mobile Machine Operators in U.S. Surface Coal Mines and 

Noise Control Techniques, by Roy C. Bartholomae and William W. Aljoe 19 

A Reduced-Noise Auger Miner Cutting Head, by William W. Aljoe and 

Mark R. Pettitt 37 

Coal Cutting Noise Control, by Mark R. Pettitt and William W. Aljoe 45 

Mantrip Noise Controls , by Roy C. Bartholomae and Thomas G. Bobick 66 

Quieted Percussion Drills, by William W. Aljoe 74 

Current Status of Load-Haul-Dump Machine Noise Control, by Thomas G. Bobick and 

Richard Madden 90 

Retrofit Noise Controls for Crushing and Screening Plants, by Terry L. Muldoon 

and Thomas G. Bobick 107 

Noise Control in Coal Preparation Plants, by Thomas G. Bobick and 

Matthew N. Rubin 120 

Overview of Bureau of Mines Hearing Protection Research, by Gerald W. Redmond 

and J. Alton Burks.. 130 





UNIT OF MEASURE 


ABBREVIATIONS USED 


IN THIS REPORT 


cm 


centimeter 


lb/ft 2 


pound per square foot 


dB 


decibel 


m 


meter 


dBA 


decibel A-weighted 


ubar 


microbar 


deg 


degree 


mm 


millimeter 


°F 


degree Fahrenheit 


min 


minute 


ft 


foot 


mph 


mile per hour 


ft 2 


square foot 


ms 


millisecond 


g 


gram 


N/m 2 


newton per square meter 


g/lb 


gram per pound 


Pa 


pascal 


h 


hour 


pet 


percent 


hp 


horsepower 


rpm 


revolution per minute 


Hz 


Hertz 


s 


second 


in 


inch 


s/in 2 


second per square inch 


in/s 


inch per second 


t/yr 


ton per year 


lb 


pound 


yd 3 


cubic yard 


lbf 


pound (force) 


yr 


year 


lb/in 


pound per inch 







NOISE CONTROL 

Proceedings: Bureau of Mines Technology Transfer Seminar, Pittsburgh, PA, 
July 24, 1984, and Denver, CO, July 26, 1984 

Compiled by Staff, Bureau of Mines 



ABSTRACT 

Overexposure to noise is a widespread, serious health hazard in the 
mining industry, and causes hearing losses to a large percentage of the 
mining population. Recognizing this fact, the Bureau has a major on- 
going research effort to address this problem area. These proceedings 
include an overview of the Bureau noise control program and summarize 
nine selected noise control research programs. 



INTRODUCTION 
By John N. Murphy 1 



Noise is often regarded as a nuisance 
rather than an occupational hazard. How- 
ever, it is known that overexposure to 
noise can cause serious hearing loss. 
The problem is especially severe in the 
mining industry. Its extent and severity 
was documented in a 1976 study by the 
National Institute for Occupational Safe- 
ty and Health (NIOSH). 2 This study found 
that underground coal miners had notice- 
ably worse hearing than the general popu- 
lation. For instance it was found that 
at age 50, over 50 pet of all coal miners 
had a hearing loss greater than 25 dB,3 
and about 30 pet had a hearing loss 
greater than 40 dB (fig. 1). Miners ex- 
perience greater hearing impairment than 
most other industrial workers because of 
their work-related overexposure to noise. 
Moreover, noise-induced hearing loss oc- 
curs gradually over many years, indi- 
viduals are not aware of it until they 
notice that they cannot effectively 
communicate with other people or hear 
safety signals in the workplace. Severe 
noise-induced hearing loss is debilitat- 
ing and cannot be cured by hearing aids. 4 

The Federal Government regulates the 
noise exposure of mine workers under the 
Federal Mine Safety and Health Amendments 
Act of 1977 (Public Law 95-164). This 
act, which supersedes the Federal Coal 

1 Research director, Pittsburgh Research 
Center, Bureau of Mines, Pittsburgh, PA. 

2 Henderson, T. L. (ed.). Survey of 
Hearing Loss in the Coal Mining Industry. 
HEW (NIOSH) Publ. 76-172, 1976, 145 pp.; 
NTIS PB 271 811 . 

■^The American Academy of Ophthalmology 
and Otolaryngology recognizes that indi- 
viduals with 25 dB hearing losses have a 
hearing handicap. 

4 For a detailed discussion on hearing 
loss see the "Federal Occupational Safety 
and Health Administration Regulation on 
Occupational Noise Hearing Conservation 
Programs" (49 FR 4078, Jan. 16, 1981). 



Mine Health and Safety Act of 1969, 
covers surface and underground operations 
of metal and nonmetal mines as well as 
coal mines. The section of the act deal- 
ing with noise is reproduced on the fol- 
lowing pages; the applicable regulations 
are found in the Code of Federal Regula- 
tions (Title 30, Chapter 1). 

Allowable noise exposure is defined in 
terms of intensity and the duration of 
exposure. Specifically, the regulations 
use the concept of noise dose to identify 
those instances when noise becomes an oc- 
cupational health problem. Table 1 shows 
the relationship between noise level and 
allowable worker exposure time mandated 
by the Federal noise regulations for 
mines . 

Exposure to a continuous noise level of 
90 dBA is permitted for no more than 8 h. 
For every 5-dBA increase in the noise 
level , the allowable exposure time is cut 
in half. As an example, no more than 4 
hours of exposure time is permitted at 95 
dBA. Also , there is an upper limit ; ex- 
posure to continuous noise levels above 
115 dBA is not permitted. 

«_ 80 



o 

a 



in 
en 
o 

_i 

<£> 

Z 

rr 

a 

X 
X 



60 



40 



UJ 



20 



i r 



1 r 



Hearing loss >25 dB 




20 30 40 50 

MINERS' AGE, yr 

FIGURE 1. - Miner hearing loss. 



TABLE 1. - Permissible noise exposure 
Duration per day, h Noise level , dBA 



8 


90 


6 

4 

3 

2 


92 
95 

97 
100 


1-1/2 


102 


1 


105 


3/4 

1/2 


107 
110 




115 



Recognizing that noise-induced hearing 
loss is a major health hazard in mining, 
the Bureau initiated a research program 
to address this problem. The first paper 
in this publication is a general overview 
of the program; one paper discusses hear- 
ing protectors. Other papers address 
selected noise control techniques for 



specific classes of mining and processing 
equipment . 

The Bureau handbook.^ distributed at 
this seminar includes a discussion of 
the major mining machinery noise prob- 
lems by machine type, currently availa- 
ble noise control technologies with cost 
estimates for their implementation, and 
noise control technologies in process 
of development. The handbook also in- 
cludes comprehensive lists of commer- 
cially available noise control products 
and materials suppliers, a bibliography, 
and case histories. 

^Bartholomae, R. C, and R, P. Parker. 
Mining Machinery Noise Control Guide- 
lines, 1983. BuMines Handbook, 1983, 87 
pp.; single copies are available from 
section of Publications, Bureau of Mines, 
4800 Forbes Avenue, Pittsburgh, PA 15 213. 



APPENDIX A: FEDERAL NOISE REGULATIONS 

The Federal government regulates the noise exposure of mine workers under 
the. Federal Mine Safety and Health Act of 1977 (P.L. 95-164). This act, which 
supersedes the previous Federal Coal Mine Safety and Health Act of 1969, covers 
surface and underground operations of metal and nonmetal mines as well as coal 
mines. These regulations are found in a government publication called the Code of 
Federal Regulations (Title 30, Chapter 1). The sections of 30 CFR 1.1 that pertain to 
noise are: 

Subchapter N Part 55 Section 55.5 Metal and Nonmetal Open Pit Mines 

56.5 Sand, Gravel, and Crushed Stone Operators 
57.5 Metal and Nonmetal Underground Mines 

Subchapter O Part 70 Subpart F Noise Standard for Underground Coal Mines 

Part 71 Subpart D Noise Standard for Surface Work Areas of 

Underground Coal Mines and Surface Coal Mines 



Only one of the three sections in Subchapter N has been reproduced; the 
others are identical. Sections of Subchapter O relating to noise are reproduced in 
their entirety. 



Physical Agents 



55.5-11 through 55.5-49 [Reserved] 

55.5-50 Mandatory, (a) No employee 
shall be permitted an exposure to noise in 
excess of that specified in the table below. 
Noise level measurements shall be made 
using a sound level meter meeting specifica- 
tions for type 2 meters contained in Ameri- 
can National Standards Institute (ANSI) 
Standard SI. 4-1971, "General Purpose 
Sound Level Meters,'' approved April 27, 
1971, which is hereby incorporated by refer- 
ence and made a part hereof, or by a dosi- 
meter with similar accuracy. This publica- 
tion may be obtained from the American 
National Standards Institute, Inc., 1430 
Broadway, New York, New York 10018, or 
may be examined in any Metal and Nonme- 
tal Mine Health and Safety District or Sub- 
district Office of the Mine Safety and 
Health Administration. 



Permissible Noise Exposures 



Duration per day. hours 


Sound level. dBA. 


slow 


of exposure 


response 




8 




90 


6 




92 


4 




95 


3 




97 


2 




100 


1 '; 
1 
1 , 




102 
105 
110 


1 t or less 




1 15 



No exposure shall exceed 115 dBA. Impact 
or impulsive noises shall not exceed 140 dB, 
peak sound pressure level. 

Note. When the daily noise exposut a is 
composed of two or more periods of noise 
exposure at different levels, their combined 
effect shall be considered rather than the 
individual effect of each. 
If the sum 



(C/r,)* (C 2 /r 2 )4 



(CJT n ) 



c-xceeds unity, then the mixed exposure 
shall be considered to exceed the permissi- 
ble exposure. C n indicates the total time of 
exposure at a specified noise level, and T n 
indicates the total time of exposure permit- 
ted at that level. Interpolation between tab- 
ulated values may be determined by the fol- 
lowing formula: 

log T- 6.322-0.0602 SL 

Where T is the time in hours and SL is the 
sound level in dBA. 

(b) When employees' exposure exceeds 
that listed in the above table, feasible ad- 
ministrative or engineering controls shall be 
utilized. If such controls fail to reduce expo- 
sure to within permissible levels, personal 
protection equipment shall be provided and 
used to reduce sound levels to within the 
levels of the table. 

[34 FR 12504, July 31, 1969. as amended at 
35 FR 3661, Feb. 25, 1970; 35 FR 18588, Dec. 
8, 1970; 39 FR 24316, July 1, 1974; 39 FR 
28433, Aug. 7, 1974; 41 FR 23612. June 10, 
1976; 43 FR 54066, Nov. 17, 1978; 44 FR 
36385, June 22, 1979] 



Subpart F — Noise Standard 

Authority: Sections 101 and 206, 83 Stat. 
745 and 765; 30 U.S.C. 801 and 846. 

Source: 36 FR 12739, July 7, 1971, unless 
otherwise noted. 

§ 70.500 Definitions. 

As used in this Subpart F, the term: 

(a) "dBA" means noise level in deci- 
bels, relative to a reference level of 20 
micro pascals, as measured by the use 
of an A-weighting and slow metering 
characteristic as specified in American 
National Standards Institute (ANSI), 
"Specification for Sound Level 
Meters," Sl.4-1971 (Type S2A). 

(b) "Noise exposure" means a period 
of time during which the noise level is 
90 or more dBA; 

(c) "Multiple noise exposure" means 
the daily noise exposure is composed 
of two or more different noise levels; 

(d) "Noise level" is the average dBA 
during a noise exposure; and, 

(e) "Qualified person" means, as the 
context requires, an individual deemed 
qualified by the Secretary and desig- 
nated by the operator to make tests 
and examinations required by this Act. 

(f) "Personal noise dosimeter" 
means equipment worn by an individu- 
al, which performs noise level mea- 
surements along with exposure time 
measurements. The circuitry of the in- 
strument is such that it automatically 
performs the computation of the mul- 
tiple noise exposure specified in 
§ 70.502. 

[35 FR 5544, Apr. 3. 1970. as amended at 43 
FR 40761. Sept. 12. 19^8] 

§ 70.501 Requirements. 

Every operator of an underground 
coal mine shall maintain the noise 
levels during each shift to which each 
miner in the active workings of the 
mine is exposed at or below the per- 



missible noise levels set forth in Table 
I of this subpart. 

Example: If a noise is recorded to be 110 
dBA then exposure shall not exceed 30 min- 
utes during an 8-hour shift. 

§ 70.502 Computation of multiple noise ex- 
posure. 

The standard will be considered to 
have been violated in the case of mul- 
tiple noise exposure where such expo- 
sure totals exceed one as computed by 
adding the total time of exposure at 
each specified level (Ci, C 2 , C 3 etc.) di- 
vided by the total time of exposure 
permitted at that level (T„ T 3 , T 3 ). 
Thus, [C,/T,] + [C a /T 2 ] + [C,/T 3 ] must 
not exceed 1. 

Example I: Exposure of 2 hours at 92 dBA 
and 1 hour at 100 dBA during an 8-hour 
shift. 

Total minutes of noise exposure at dBA 
level/Total minutes of permissible noise 
exposure at dBA level [120 min./360 
min. + 60 min./120 min.] = y 8 + Vfe 

= % + 3/ 6 = 5 / 6 

The sum of the fractions does not exceed 
one; hence the exposure for the shift would 
not violate the standard. 

Example II: Exposure of 3 hours at 95 
dBA and 1 hour at 100 dBA during an 8 
hour shift. 

% + i/ 2 = % + % = s/ 4 

The sum of the fractions exceeds one; 
hence the exposure for the shift would vio- 
late the standard. 

§ 70.503 Noise exposure measurements; 
general. 

Every coal mine operator shall take 
accurate readings of the noise levels to 
which each miner in the active work- 
ings of the mine is exposed during the 
performance of the duties to which he 
is normally assigned. 

[36 FR 12739. July '., 1971, as amended at 43 
FR 40761. Sept 12. 1978] 



§ 70.504 Noise exposure measurements; by 
whom done. 

The noise exposure measurements 
required by this Subpart F shall be 
taken by, or as directed by, a person 
who has mec the minimum require- 
ments set forth in § 70.504-1, and has 
been certified by the Assistant Secre- 
tary of Labor for Mine Safety and 
Health, Mine Safety and Health Ad- 
ministration as qualified to take noise 
exposure measurements as prescribed 
in this Subpart F. 

[36 FR 12739, July 7, 1971, as amended at 43 
FR 12319, Mar 24, 1978; 43 FR 40761. Sept. 
12. 1978; 43 FR 43458, Sept. 26. 1978] 

§ 70.504-1 Persons qualified to measure 
noise levels; minimum requirements. 

The following persons shall be con- 
sidered qualified to take noise expo- 
sure measurements as prescribed in 
this Subpart F; 

(a) Any person who has been certi- 
fied by the Mine Safety and Health 
Administration as an instructor in 
noise measurement training programs; 

(b) Any person who has satisfactori- 
ly completed a noise training course 
conducted by the Mine Safety and 
Health Administration and has been 
certified by the Administration as a 
qualified person; and, 

(c) Any person who has satisfactori- 
ly completed a noise training course 
approved by the Mine Safety and 
Health Administration and has been 
certified by the Administration as a 
qualified person. 

[36 FR 12739, July 7, 1971. as amended at 43 
FR 40761, Sept. 12, 1978] 

§ 70.504-2 Certification of qualified per- 
sons by the Mine Safety and Health 
Administration. 

Upon a satisfactory showing that a 
person has met the minimum require- 
ments for taking noise exposure mea- 
surements set forth in § 70.504-1, the 
Mine Safety and Health Administra- 



tion shall certify that such person has 
the ability and capacity to conduct 
tests of the noise exposure in a coal 
mine and to report and certify the re- 
sults of such tests to the Secretary 
and the Secretary of Health, Educa- 
tion, and Welfare. 

[36 FR 12739, July 7, 1971, as amended at 43 
FR 40761, Sept. 12, 1978] 

§ 70.505 Noise exposure measurement 
equipment. 

Noise exposure measurements shall 
be taken only with equipment which is 
approved by the Mine Safety and 
Health Administration as permissible 
electric face equipment under the pro- 
visions of Part 18 of this chapter and 
which in the case of sound level 
meters, meets American National 
Standards Institute (ANSI), "Specifi- 
cation for Sound Level Meters,' 1 SI. 4- 
1971 (Type S2A), or in the case of per- 
sonal noise dosimeters, has been found 
to be acceptable by the Mine Safety 
and Health Administration. 

[43 FR 40761, Sept. 12, 1978] 

§ 70.506 Noise exposure measurement pro- 
cedures; instrument setting; calibra- 
tion. 

(a) Noise exposure measurements 
made with sound level meters shall 
conform to the following: 

(1) Noise exposure measurements 
shall be made at locations where the 
noise it typical of that entering the 
ears of the miner whose exposure is 
under consideration. 

(2) Five measurements shall be made 
for each type of noise exposure pro- 
ducing operation to which the miner 
under consideration is exposed. 

(3) Each measurement shall be made 
by observing the A-scale readings for 
30 seconds and recording the noise 
level. 

(4) The average of the five noise 
level measurements shall be consid- 
ered as the noise level measurement 



which is representative of the oper- 
ation. 

(5) Where different and distinct 
noise levels occur at various phases of 
an operation, noise exposure measure- 
ments shall be made in accordance 
with this section for each distinct 
phase. 

(6) The noise levels and the estimat- 
ed length of time the miner is exposed 
to each level during a normal work 
shift shall be reported for the oper- 
ation. 

(b) Noise exposure measurements 
made with personal noise dosimeters 
shall conform to the following: 

(1) For the miner whose noise expo- 
sure is under consideration, noise ex- 
posure measurements shall be made 
with the personal noise dosimeter mi- 
crophone located at the top of the 
shoulder, midway between the neck 
and the end of the shoulder with the 
microphone pointing in a vertical 
upward direction in accordance with 
the diagram shown below: 



Drawing Illustrating 
Proper Noise 
Dosimeter 
Microphone 
Placement 



Mid-Distance 

on Shoulder Blade 

/ 

! — --• — 

Grid of 
Microphone 

Microphone 

Cord 

Noise Dosimeter 




(2) To the extent practical, the per- 
sonal noise dosimeter instrument case 
and microphone cable shall be posi- 



tioned underneath exterior clothing so 
as to minimize potential safety prob- 
lems and damage to the instrument. 
The microphone shall not be covered 
by clothing. 

(3) The personal noise dosimeter 
shall be worn by the miner whose 
noise exposure is under consideration 
for an entire normal work shift and 
the accumulated per centum of the 
noise exposure shall be reported. 

(c) Noise exposure measurement in- 
struments specified in § 70.505 shall be 
set to operate with the A-weighted 
network and slow response. 

(d)(1) Sound level meters and per- 
sonal noise dosimeters used by an op- 
erator in fulfilling the requirements of 
this subpart shall be acoustically cali- 
brated in accordance with the manu- 
facturer's instructions before and 
after each shift on which the meter is 
used. 

(2) Sound level meters and personal 
noise dosimeters used by an author- 
ized representative of the Secretary 
shall be acoustically calibrated in ac- 
cordance with the manufacturer's 
instructions or by another equivalent 
procedure before and after each shift 
on which the meter is used. 

(3) Personal noise dosimeters shall 
be recalibrated annually, including, as 
a minimum, the following: 

(i) Visual inspection of the micro- 
phone for any foreign matter or 
damage, 

(ii) Comparison of the dosimeter, at 
1000 Hz, with a laboratory type con- 
densor microphone of known sensitiv- 
ity, and 

(iii) Frequency response testing in a 
free or diffused field where the sound 
field is established using a laboratory 
type condensor microphone of known 
sensitivity. 

(4) A document containing the date 
of the annual recalibration of each 
personal noise dosimeter and the 



names of the individual and organiza- 
tion performing the calibration shall 
be kept on file at each mine office. 

(e)(1) Acoustical calibrators which 
are used to calibrate sound level 
meters and personal noise dosimeters 
shall be recalibrated once a year using 
a laboratory type condensor micro- 
phone of known sensitivity as deter- 
mined by a National Bureau of Stand- 
ards calibration. 

(2) A document containing the date 
of the annual calibration of each 
acoustical calibrator and the names of 
the indivdual and organization per- 
forming the calibration shall be on file 
at each mine office. 

[43 FR 40761. Sept. 12. 1978. as amended at 
43 FR 50678. Oct 31. 1978] 

§ 70.507 Initial noise exposure survey. 

On or before June 30, 1971, each op- 
erator shall: 

(a) Conduct, in accordance with this 
subpart, a survey of the noise levels to 
which each miner in the active work- 
ings of the mine is exposed during his 
normal work shift; and, 

(b) Report and certify to the Mine 
Safety and Health Administration, 
and the Department of Health, Educa- 
tion, and Welfare, the results of such 
survey using the Coal Mine Noise Data 
Report, Figure 1. Reports shall be sent 
to: 

Division of Automatic Data Processing. 
Mining Enforcement and Safety Adminis- 
tration, Building 53, Denver Federal 
Center, Colo. 80225. 

[36 FR 12739. July 7. 1971, as amended at 43 
FR 40762. Sept 12, 1978] 

§ 70.508 Periodic noise exposure survey. 

(a) At intervals of at least every 6 
months after June 30, 1971, but in no 
case shall the interval be less than 3 
months, each operator shall conduct, 
in accordance with this subpart, peri- 
odic surveys of the noise levels to 



which each miner in the active work- 
ings of the mine is exposed and shall 
report and certify the results of such 
surveys to the Mine Safety and Health 
Administration, and the Department 
of Health, Education, and Welfare, 
using the Coal Mine Noise Data 
Report Form. Reports shall be sent to: 

Division of Automatic Data Processing, 
Mining Enforcement and Safety Adminis- 
tration, Building 53, Denver Federal 
Center. Colo. 80225. 

(b) Where no A-scale reading record- 
ed for any miner during an initial or 
periodic noise exposure survey exceeds 
90 dBA, the operator shall not be re- 
quired to survey such miner during 
any subsequent periodic noise level 
survey required by this section: Pro- 
vided, however, That the name and 
job position of each such miner shall 
be reported in every periodic survey 
and the operator shall certify that 
such miner's job duties and noise ex- 
posure levels have not changed sub- 
stantially during the preceding 6- 
month period. 

[36 FR 12739. July 7, 1971. as amended fct 43 
FR 40762. Sept. 12, 1978] 

§ 70.509 Supplemental noise exposure 
survey; reports and certification. 

(a) Where the certified results of an 
initial noise exposure survey conduct- 
ed in accordance with § 70.507, or a pe- 
riodic noise exposure survey conducted 
in accordance with § 70.508, show that 
any miner in the active workings of 
the mine is exposed to a noise level in 
excess of the permissible noise level 
prescribed in Table I, the operator 
shall conduct a supplemental noise ex- 
posure survey with respect to each 
miner whose noise exposure exceeds 
this standard. This survey shall be 
conducted within 15 days following no- 
tification to the operator by the Mine 
Safety and Health Administration to 
conduct such survey. 



10 



(b) Supplemental noise exposure 
surveys shall be conducted by taking 
noise exposure measurements in ac- 
cordance with § 70.506, however, noise 
exposure measurements shall be taken 
during the entire period of each indi- 
vidual operation to which the miner 
under consideration is actually ex- 
posed during his normal work shift. 

(c) Each operator shall report and 
certify the results of each supplemen- 
tal noise level survey conducted in ac- 
cordance with this section to the Mine 
Safety and Health Administration and 
the Department of Health, Education, 
and Welfare using the Coal Mine 
Noise Data Report Form to record 
noise level readings taken with respect 
to all operations during which such 
measurements were taken. 

(d) Supplemental noise exposure 
surveys shall, upon completion, be 
mailed to: 

Division of Automatic Data Processing, 
Mine Safety and Health Administration, 
Building 53, Denver Federal Center, Colo. 
80225. 

[36 FR 12739, July 7, 1971, as amended at 43 
FR 40762. Sept. 12, 1978] 

§ 70.510 Violation of noise standard; 
notice of violation; action required by 
operator. 

(a) Where the results of a supple- 
mental noise exposure survey conduct- 
ed in accordance with § 70.509 show 
that any miner in the active workings 
of the mine is exposed to noise levels 
which exceed the permissible noise 
levels prescribed in Table I, the Secre- 
tary shall issue a notice to the opera- 
tor that he is in violation of this sub- 
part. 

(b) Upon receipt of a Notice of Viola- 
tion issued pursuant to paragraph (a) 
of this section, the operator shall: 

(1) Institute promptly administra- 
tive and/or engineering controls neces- 



sary to assure compliance with the 
standard. Such controls may include 
protective devices other than those de- 
vices or systems which the Secretary 
or his authorized representative finds 
to be hazardous in such mine. 

(2) Within 60 days following the is- 
suance of any Notice of Violation of 
this subpart, submit for approval to a 
joint Mine Safety and Health Admin- 
istration-Health, Education, and Wel- 
fare committee, a plan for the admin- 
istration of a continuing, effective 
hearing conservation program to 
assure compliance with this subpart, 
including provision for: 

(i) Reducing environmental noise 
levels; 

(ii) Personal ear protective devices to 
be made available to the miners; 

(iii) Preemployment and periodic au- 
diograms. 

(3) Plans required under subpara- 
graph (2) of this paragraph shall be 
submitted to: 

Assistant Administrator. Coal Mine Health 
and Safety, Mine Safety and Health Ad- 
ministration, Department of Labor, 4015 
Wilson Boulevard. Arlington. Va. 22203. 

Tablf I— Permissible Noise Exposures 



Duration per 
day {hours) 

8 

6 

4 

3 

2 

1 'a 



< or iess 



Noise level 

(dBA) 

90 

92 

95 

97 
100 
102 
105 
107 
110 
115 



[35 FR 5544. Apr. 3. 1970. as amended at 43 
FR 12319. Mar. 24. 1978: 43 FR 40762. Sept. 
12. 1978] 



11 



Chapter I — Min* Safety and Health Admin. 



§70.510 



(Submit one form for e.i. h miner) F 


gure 1 


COAL MINE NO I St. DMA REPORT 

Date: / / 

mo . day yr . 

Company: Mine Name 


Mine I . D. Number : 






Section/Pit Number: 
Miners Name : 





Miners SSN: 






Occupa t ion Code : 




Initial Periodi, 


Supp 1 emen t <i I 




Hearing Protective Devi.r I'm 
Yes No If Yes 


'K" Value 




Equipment in Operation: 
Manuf ac t urer 




Type 


Model Number 




Serial or Company Number 




Dosimeter Reading: 




Measurement Time tin Minutes 


. : 


Operat ions 
(Loading, Tramm 1 ng , Et i . i 


Noise Level Minutes 
dBA Average Exposure 











































I 



§70.511 Incorporation by reference. 

In accordance with 5 U.S.C. 
552(a)(1), the technical publication, 
"Specification for Sound Level 
Meters", Sl.4-1971 (Type S2A), issued 
by the American National Standards 
Institute (ANSI), April 27, 1971, refer- 
enced in this subpart F is hereby in- 
corporated by reference and made a 
part hereof. The incorporated techni- 
cal publication is available for exami- 
nation at MSHA, 4015 Wilson Blvd., 
Arlington, Va. 22203; the National In- 
stitute for Occupational Safety and 
Health, 5600 Fishers Lane, Rockville, 
Md. 20857; each Coal Mine Health and 
Safety District and Subdistrict Office 
and the Federal Register library. In 
addition, copies of the document can 
be purchased from the American Na- 
tional Standards Institute (ANSI), 
1430 Broadway, New York, N.Y. 10018. 

[43 FR 40762. Sept. 12. 1978] 



Signature of Qualified Person :_ 



12 



Subpart D — Noise Standard 

§71.300 Noise standard; general require- 
ments. 

Each operator of an underground 
coal mine and each operator of a sur- 
face coal mine shall, during each shift, 
maintain the noise level to which each 
miner in each surface installation and 
at each surface worksite is exposed at 
or below the maximum noise exposure 
level prescribed in Subpart F, Part 70 
of this Subchapter O. 

§ 71.301 Measurement of noise levels. 

Each operator shall measure the 
noise level to which each miner is ex- 
posed in each surface installation and 
at each surface worksite in the 
manner prescribed in Subpart F, Part 
70, of this Subchapter O. 

§ 71.302 Initial noise exposure survey. 

On or before November 13, 1974 
each operator shall: 

(a) Conduct, in accordance with this 
subpart, a survey of the noise levels, to 
which each miner in each surface in- 
stallation and at each surface worksite 
is exposed during his normal work 
shift; and, 

(b) Report and certify to the Mine 
Safety and Health Administration and 
the Department of Health, Education, 
and Welfare, the results of such 
survey using the Coal Mine Noise Data 
Report. (See Figure 1, Part 70 of this 
subchapter.) Reports shall be sent to: 

Division of Automatic Data Processing, Post 
Office Box 25407, Building 41, Denver 
Federal Center, Denver, CO 80225. 

[39 FR 17102, May 13, 1974, as amended at 
43 FR 40764. Sept. 12. 1978] 

§ 71.303 Periodic noise exposure survey. 

(a) At intervals of at least every 6 
months, after November 13, 1974 each 
operator shall conduct periodic sur- 



veys of the noise levels to which each 
miner in each surface installation and 
at each surface worksite is exposed 
and shall report and certify the results 
of such surveys to the Mine Safety 
and Health Administration and the 
Department of Health, Education, and 
Welfare, using the Coal Mine Noise 
Data Report Form. The interval be- 
tween each survey shall not be less 
than 3 months. Reports shall be sent 
to: 

Division of Automatic Data Processing, Post 
Office Box 25407, Building 41, Denver 
Federal Center, Denver, CO 80225. 

(b) Where no A-scale reading record- 
ed for any miner during an initial or 
periodic noise exposure survey exceeds 
90 dBa, the operator shall not be re- 
quired to survey such miner during 
any subsequent periodic noise expo- 
sure survey required by this section: 
Provided, however, That the name and 
job position of each such miner shall 
be reported in every periodic survey 
and the operator shall certify that 
such miner's job duties and noise ex- 
posure levels have not changed sub- 
stantially during the preceding 6- 
month period. 

[39 FR 17102, May 13, 1974, as amended at 
43 FR 40764, Sept. 12. 1978] 

§71.304 Supplemental noise exposure 
survey; reports and certification. 

(a) Where the certified results of an 
initial noise exposure survey conduct- 
ed in accordance with § 71.302 or a pe- 
riodic noise exposure survey conducted 
in accordance with § 71.303 indicate 
that any miner may be exposed to a 
noise exposure in excess of the permis- 
sible noise exposure, the operator 
shall conduct a supplemental noise ex- 
posure survey with respect to each 
miner whose noise exposure exceeds 
this standard. This survey shall be 
conducted within 15 days following no- 
tification to the operator by the N'ine 



13 



Safety and Health Administration to 
conduct such survey. 

(b) Supplemental noise exposure 
surveys shall be conducted by taking 
noise exposure measurements in ac- 
cordance with § 70.506 of this Sub- 
chapter O; however, noise exposure 
measurements shall be taken of each 
individual operation to which the 
miner under consideration is actually 
exposed during his normal work shift 
and the duration of each such expo- 
sure shall be recorded. 

(c) Each operator shall report and 
certify the results of each supplemen- 
tal noise exposure survey conducted in 
accordance with this section to the 
Mine Safety and Health Administra- 
tion and the Department of Health, 
Education, and Welfare using the Coal 
Mine Noise Data Report Form to 
record noise level readings taken with 
respect to all operations during which 
such measurements were taken. 

(d) Supplemental noise exposure 
surveys shall, upon completion, be 
mailed to: 

Division of Automatic Data Processing. Post 
Office Box 25407, Building 41, Denver 
Federal Center, Denver, CO 80225. 

[39 FR 17102, May 13, 1974, as amended at 
43 FR 40764, Sept. 12, 1978] 



§ 71.305 Violation of noise standard; 
notice of violation; action required by 
operator. 

(a) Where the results of a supple- 
mental noise exposure survey conduct- 
ed in accordance with § 71.304 indicate 
that any miner is exposed to noise 
levels which exceed the permissible 
noise levels, the Secretary shall issue a 
notice to the operator that he is in vio- 
lation of this subpart. 

(b) Upon receipt of a notice of viola- 
tion issued pursuant to paragraph (a) 



of this section, the operator shall: 

(1) Institute, promptly, administra- 
tive and/or engineering controls neces- 
sary to assure compliance with the 
standard. Such controls may include 
protective devices other than those de- 
vices or systems which the Secretary 
or his authorized representative finds 
to be hazardous in such mine. 

(2) Within 60 clays following the is- 
suance of the first notice of violation 
of this subpart, submit for approval to 
a joint Mine Safety and Health Ad- 
ministration/Health, Education, and 
Welfare committee, a plan for the ad- 
ministration of a continuing, effective 
hearing conservation program to 
assure compliance with this subpart, 
including provision for: 

(i) Reducing environmental noise 
levels; 

(ii) Personal ear protective devices to 
be made available to the miners; 

(iii) Preplacement and periodic au- 
diograms. 

(iv) Those administrative and engi- 
neering controls that it has instituted 
to assure compliance with the stand- 
ard. 

(3) Plans required under subpara- 
graph (2) of this paragraph shall be 
submitted to: 

Division of Automatic Data Processing, Post 
Office Box 25407, Building 41, Denver 
Federal Center, Denver. CO 80225. 

(c) Within 30 days following the is- 
suance of any subsequent notice of 
violation of this subpart, the operator 
shall submit in writing: 

(i) A statement of the manner in 
which the plan is intended to prevent 
the violation or 

(ii) A revision to its plan to prevent 
similar future violations. 

[39 FR 17102. May 13, 1974, as amended at 
43 FR 40764, Sept. 12, 1978] 



14 



BUREAU OF MINES OCCUPATIONAL NOISE CONTROL PROGRAM 
By J. Harrison Daniel 1 



Excessive noise levels represent the 
most widespread occupational health prob- 
lem in the mining and ore processing in- 
dustries. Virtually every worker in un- 
derground and surface mines and in crush- 
ing and grinding ore processing mills is 
exposed to levels of noise that can cause 
permanent reduction in the ability to 
hear. Because of continuing developments 
in mining methods and new equipment de- 
signs, as well as expanding production 
goals , the noise levels in both under- 
ground and surface mines are becoming an 
increasing threat to the health and safe- 
ty of the Nation's miners. With more 
than 500,000 persons now working in un- 
derground and surface mines and in prep- 
aration plants and mills , the seriousness 
of noise hazards cannot be overestimated. 

The goal of the Bureau's noise control 
program is to reduce noise exposure of 
all miners and ore processing personnel 
to within the limits prescribed in -the 
Federal Mine Safety and Health Amendments 
Act of 1977 without affecting increased 
production demands. 

The 1977 act limits noise exposure to a 
90-dBA level (dBA is a relative measure 
of sound pressure, weighted to match the 
frequency response of the human ear) for 
an 8-h duration. As noise levels in- 
crease, permissible exposure durations 
decrease. For example, the ceiling limit 
of 115 dBA requires that exposure be lim- 
ited to 15 min or less. Noise exposure 
levels over 115 dBA are not permissible. 

Extensive surveys have shown that the 
noise in underground and surface mines 
often exceeds the permissible exposures. 
In coal mining, most major equipment, in- 
cluding continuous miners, loaders, 
stoper drills, and cleaning plant appa- 
ratus, has been identified as contribut- 

1 Staff engineer, Division of Health and 
Safety Technology, Bureau of Mines, Wash- 
ington; D.C. 



ing to excessive noise exposure. Figure 
1 shows typical noise levels and operat- 
ing times per shift of underground coal 
mining machines. 

In metal and nonmetal mining, major 
equipment contributing to excessive noise 
levels includes bulldozers , rock drills , 
channel burners used in quarrying, load- 
haul-dump vehicles , and ore processing 
equipment. Although personal hearing 
protectors such as ear muffs or earplugs 
are widely used, these are only interim 
or short-term solutions to noise over- 
exposure problems. Long-term solutions 
require that the noise be reduced at its 
source. 

The following are the objectives of 
Bureau programs to develop noise abate- 
ment technology : 

To determine major noise sources in 
underground and surface mines and 
mineral processing plants. 

To develop and evaluate both 
factory-installed noise controls for 
equipment and processing operations 
and noise-control field modifications 
to existing equipment. 

To provide more accurate noise ex- 
posure measuring techniques and in- 
strumentation. 

To establish a procedure to evalu- 
ate the actual noise attenuation of 
personal hearing protectors worn by 
miners. 

To develop concepts and design 
techniques that will provide inher- 
ently quieter mining equipment and 
operations for the future. 

To make available to industry the 
technical knowledge needed to select, 
design, and use the most effective 
noise-control measures. 



15 



I25i r 



< 

CD 



UJ 

> 

UJ 



UJ 

to 




85 — 



80 



g machine 



Shuttle car 
s Coal drill 

J i L 



j_ 



40 80 120 160 

OPERATING TIME, min 



200 



FIGURE 1. - Typical noise levels and operat- 
ing times per 8-h shift of underground coal min- 
ing machines. 

The Bureau's main research efforts have 
focused on reduction of mining noise at 
the source by identifying specific noise 
sources and developing abatement technol- 
ogy and methods. Because mining ma- 
chinery is expensive and much equipment 
now in use is not due to be replaced for 
many years , a large part of the research 
program has involved development of 
"retrofit" noise abatement techniques and 
materials that can be used to modify ex- 
isting machinery. However, long-term 
work increasingly will be concerned with 
factory integration of control measures 
and the design of quieter machines. In- 
creased emphasis will also be given to 
the development of new mining methods 



that will offer advantages in noise con- 
trol over the established methods in use 
today. 

Where possible, the Bureau seeks the 
advice and cooperation of both equipment 
manufacturers and operators experienced 
in using the equipment. Such cooperation 
is especially important in view of the 
high cost and limited availability of 
mining machinery. Cost sharing agree- 
ments with industry and the cooperation 
of mine operators are essential to the 
program. Over the past 5 yr at least 25 
projects required the cooperation of over 
100 mines for noise surveys , data collec- 
tion, and operational testing of Bureau 
developed or modified equipment. 

Technology and techniques that are de- 
veloped are applied to test machines pro- 
vided by mine operators or machinery man- 
ufacturers and evaluated under production 
conditions. Noise controls developed 
must be cost effective, must be readily 
adaptable to existing machines and opera- 
tions, and must not reduce machinery ef- 
ficiency or lower production levels. 

The Bureau has been actively conducting 
noise abatement research and development 
programs both in-house and through Bureau 
funded contract studies since the passage 
of the Federal Coal Mine Health and Safe- 
ty Act of 1969. This act was later 
amended by the 1977 act. Both acts cite 
the allowable noise exposure levels given 
in table 1 in the introduction to these 
proceedings. The research began in 1970 
with an initial contract study of hearing 
protection in underground mines and an 
in-house project to develop a muffler 
jacket for a percussion drill. Through 
the mid-1970 1 s and late 1970' s, the re- 
search program grew substantially and 
consisted almost entirely of contract 
research until 1981 when the percentage 
of in-house work began to increase sig- 
nificantly. This increase in the in- 
house program was accompanied with over- 
all Federal budget reductions. 

The majority of the research done in 
the 1970' s consisted of noise surveys and 
analyses to determine what were the major 



16 



noise problems in both underground and 
surface mining, what the principal noise 
sources were for specific equipment and 
systems, and what remedial measures could 
be taken to alleviate the noise problem. 
In the late 1970' s the emphasis was on 
retrofit noise controls of existing 
equipment. These retrofit modifications 
were designed to be installed on equip- 
ment either at a minesite or at an equip- 
ment rebuild or overhaul facility. These 
early efforts resulted in two important 
accomplishments. First, they provided 
significant short-term and cost-effective 
noise control measures. Second, they es- 
tablished an equipment noise level data 
base on which to plan long-term design 
research to incorporate noise control 
technology into the design and manufac- 
ture of mining equipment. 

Thus, by 1981 the nature of the Bu- 
reau's noise control research had changed 
in a fundamental way. The emphasis now 
and in the coming years is on long-term 
basic research to develop and assure ade- 
quate noise control technology for future 
mining machinery concepts. Closely re- 
lated to this goal is the development of 
noise controls that can be incorporated 
into equipment when it is sent to rebuild 
shops for scheduled maintenance. This is 
very cost effective and the Bureau has 
had recent success in working with the 
private sector in this area. 

Simultaneous with this change to long- 
term research is an increase in the 
amount of work performed in-house. Since 
the late 1970' s the Bureau has steadily 
built in-house expertise and facilities 
to the point that research that was pre- 
viously performed on contract can now be 
accomplished in-house. During 1984, a 
noise test facility was constructed at 
the Bureau's Pittsburgh (PA) Research 
Center, which will allow the Bureau's 
technical staff to conduct equipment 
noise control research. 

In summary, from 1970 to the 1980' s, 
the noise control research program has 
changed from a contract program with 
goals of establishing the major noise 
problems and determining what could be 



done in the short term to provide immedi- 
ate relief, to a principally in-house 
program to perform long-term research 
that will provide permanent solutions. 
The in-house test facility constructed 
during 1984 will enable the Bureau to 
cost effectively evaluate engineering 
noise controls, to redesign equipment 
components , and to provide a technically 
sound basis for the Bureau-funded con- 
tract program to complement the in-house 
efforts. 

The success of any health-related re- 
search program is difficult to measure. 
Significant trends take many years to 
evaluate, and there are many parameters 
that affect the results. It is also dif- 
ficult to select a realistic measure with 
which to determine the results. Figure 2 
illustrates one method of measuring the 
success of the Bureau's noise research in 
metal and nonmetal mines and mills . The 
figure is a summary plot of 88,498 Mine 
Safety and Health Administration (MSHA) 
noise dosimeter readings that were taken 
in metal and nonmetal mines and mills 
since 1974. The plot of the percentage 
of dosimeter readings that represent 
noise levels greater than Federal regula- 
tions allow, shows a gradual decrease 
from 1975 through 1982. 

The noise dosimeter measures the cumu- 
lative noise exposure of workers over a 
working shift and thus records a noise 
dose reading. A meter reading of 1.00, 
or 100 pet, represents a dose equal to 
the maximum allowable noise exposure un- 
der current regulations and is referred 
to here as a threshold limit value (TLV) . 
A reading of over 100 pet means the work- 
er was overexposed, and a reading of less 
than 100 pet means the worker was exposed 
to less than the allowable maximum. 

The left-hand axis of the plot in fig- 
ure 2 shows the percent of samples that 
exceed the TLV in each year. The right- 
hand axis of the plot shows the geometric 
mean concentration in percent for each 
year. The two lines shown are simple 
linear regression lines depicting the 
general trend in exposure for all years. 
A log-probability plot of the dosimeter 



17 



80 



.Overall trend for 
concentration 



KEY 

>TLV, pet 
Concentration, pet 



o 

CL 




80 



1974 1975 1976 1977 1978 1979 1980 1981 1982 



FIGURE 2. - Summary plot of MSHA noise dosimeter readings in metal and nonmetal mines and mills 
since 1974. 



data reveals that the concentrations 
recorded in percent follow a log-normal 
distribution, that is the plot looks like 
a straight line, thus the geometric mean 
and standard deviation are appropriate 
measures of central tendency and disper- 
sion. 

The research objectives of determining 
the major noise sources in the mining in- 
dustry, developing and evaluating retro- 
fit noise control treatments for mining 
equipment, and providing more accurate 
noise exposure measuring instrumentation, 
have largely been accomplished. Current 
program emphasis is on designing and de- 
veloping concepts that will result in in- 
herently quieter equipment and mining 
operations and on evaluating the actual 
noise attenuation provided by personal 
hearing protectors. The current program 



focus is on the following four major re- 
search areas: 

Coal Extraction . — This area of research 
will develop noise control technology for 
coal extraction equipment and will trans- 
fer this technology to equipment manufac- 
turers for incorporation into newly manu- 
factured equipment. Long-term projects 
on both continuous miners and longwall 
systems were initiated several years ago 
and have reached milestones for scheduled 
completion by 1987. These projects are 
of particular significance since together 
these mining techniques account for ap- 
proximately 70 pet of the U.S. coal pro- 
duction. Emphasis is on a systems ap- 
proach, so that noise reduction of the 
major noise sources of each machine is 
being accomplished. For the continuous 
miner, noise from the chain conveyor has 



18 



been successfully reduced and current 
work is concentrating on designing quiet- 
er cutting heads. In 1984, in-mine test- 
ing of a continuous miner that has a 
noise-controlled chain conveyor and cut- 
ting head will be conducted. In the 
longwall system, coal cutting was identi- 
fied as the major source of noise. New 
cutting drum designs are being developed 
and in-mine testing of a prototype should 
commence in 1985. 



Percussion Drills. — Percussion 

some 



drills 
of the 



expose their operators to 
highest levels of noise in the mining in- 
dustry, often to intolerable levels of 
Because there are over 60,000 
in use in U.S. under- 
significance of this 



120 dBA. 

percussion drills 
ground mines , the 
problem is clear. 



The Bureau's work on 



percussion drills is composed of three 
efforts: (1) jumbo mounted drills used 
principally for drilling blastholes in 
hard-rock mines and tunnels, (2) hand- 
held hard-rock drills, and (3) drill 
steel design concepts for attenuating 
noise generated and radiated by the drill 
steel. In 1984, these efforts will be in 
various stages of development from proto- 
type fabrication of the jumbo drill to 
final design formulation of a concen- 
trically enclosed drill steel. 

Mobile Equipment . — Noise control tech- 
niques have been developed for many of 
the mobile equipment types used in mining 
operations. To date, successful controls 
have been applied to personnel carriers , 
auger-type continuous miners , chain-type 
conveyor systems , and diesel-engine- 
powered vehicles including utility vehi- 
cles, dozers, front -end . loaders, and 



load-haul-dump machines. These programs 
will continue with less emphasis and with 
the principal research being conducted at 
the Bureau's noise test facility. Empha- 
sis in 1984 and the future will be di- 
rected to redesign efforts pertaining to 
specific equipment types. 

Hearing Protectors . — Although the use 
of hearing protectors is permitted by 
MSHA only when other means of noise con- 
trol are not available, these devices 
represent an important protective measure 
because of the high noise levels of some 
mining equipment such as percussion 
drills. The Bureau is establishing a 
capability to investigate methods of 
evaluating hearing protector performance 
that can be used in the field. Conven- 
tionally, the effectiveness of ear muffs 
is determined by a psychophysical labora- 
tory technique that measures the hearing 
threshold for a person wearing the ear 
muffs. However, there is some question 
as to whether this measurement is equiva- 
lent to a physical measurement of the 
noise attenuation provided by the same 
ear muffs. 

The final result of the protector pro- 
gram is to develop a method for evaluat- 
ing ear protector effectiveness that is 
simpler, less costly, and less time con- 
suming to perform than the conventional 
audiometric approach. The results will 
not only provide workers with the actual 
protection they can expect from wearing 
hearing protectors , but also will allow 
for realistic noise reduction goals to be 
established for the equipment and opera- 
tions that exhibit noise levels in excess 
of 115 dBA. 



19 



THE NOISE EXPOSURES OF MOBILE MACHINE OPERATORS IN U.S. SURFACE COAL 
MINES AND NOISE CONTROL TECHNIQUES 

By Roy C. Bartholomae 1 and William W. Aljoe 2 



ABSTRACT 



The reduction of the noise exposure of 
operators of mobile surface mine equip- 
ment has been a primary objective of the 
Bureau of Mines. Approximately 25,000 
mobile equipment operators in U.S. sur- 
face coal mines (45 pet of the operators) 
are overexposed to noise. These mobile 
machines include bulldozers , front-end 



loaders , haulers , shovels , draglines , 
trucks, scrapers, drills, and motor 
graders. Bull-dozers and front-end load- 
ers cause about two-thirds of the noise 
overexposures. The most cost-effective 
operator noise exposure control was de- 
termined to be acoustical cabs. 



INTRODUCTION 



Many mobile machines used in U.S. sur- 
face coal mines produce noise levels 
higher than those permitted by the Fed- 
eral Coal Mine Health and Safety Act of 
1969 (Public Law 91-173) and the Federal 
Mine Safety and Health Amendments Act of 
1977 (Public Law 95-164). Recognizing 
this problem, the Bureau sponsored proj- 
ects to identify and control noise levels 
from these machines. The first project 



was a census of the types and number of 
mobile machines in surface coal mines 
0_).3 This project involved measurements 
of noise generated by mobile machines and 
an estimate of the total operator over- 
exposure (2^) . Cost-effective noise con- 
trol techniques were then developed and 
proven on bulldozers and front-end load- 
ers (3-6) . 



MACHINE CENSUS 



Results from a combination of question- 
naires and extrapolations show there were 
approximately 38,500 mobile machines in 
use at U.S. surface coal mines in 1977. 
Extrapolations were required because al- 
though a questionnaire was mailed to 
every mine address on Mine Safety and 
Health Administration (MSHA) and Bureau 
lists, not all mines responded. Two 
methods of extrapolation were used inde- 
pendently; one was based on production 
and the other was based on survey re- 
sponse rate. Both methods yielded com- 
parable results (1) . 

Table 1, which lists the machines in 
order of the number in use, shows that 
two types dominate. Heavy track dozers 

1 Supervisory electrical engineer. 
2 Mining engineer. Pittsburgh Research 
Center, Bureau of Mines, Pittsburgh, PA. 



(>150 hp) are the most numerous, account- 
ing for more than 27 pet of all machines. 
They are followed by heavy wheel front- 
end loaders , which account for more than 
16 pet. Together these two types account 
for over 43 pet of all machines used in 
surface coal mines. All types of dozers 
combined form nearly 30 pet of the total 
population, and all types of loaders form 
nearly 20 pet, together accounting for 
nearly one-half of all machines used in 
surface coal mines. 

Table 2 lists the predominant manufac- 
turers of each major machine category, 
the predominant models in use, and the 
percentage of these models in each cate- 
gory. For example, the table shows that 

^Underlined numbers in parentheses re- 
fer to items in the list of references 
preceding the appendix to this paper. 



20 



TABLE 1. - Ranking of machine types on basis of numbers in use 





Percent — 


Machine 


Rank 


Percent — 




Rank 


Of 


Cumu- 


Of 


Cumu- 


Machine 




total 


lative 






total 


lative 




i.: 


27.3 


27.3 


Dozer, track, >150 hp. 


10.. 


3.0 


90.9 


Scraper. 


2.. 


16.4 


43.7 


Loader, wheel, >150 hp. 


11.. 


2.5 


93.4 


Loader, wheel, <150 hp. 


3.. 


14.6 


58.3 


Hauler. 


12.. 


1.7 


95.1 


Dozer, track, <150 hp. 


4.. 


7.6 


65.9 


Truck , highway . 


13.. 


1.3 


96.4 


Shovel and dragline, 


5.. 


7.5 


73.4 


Shovel and dragline, 








electric, <30 yd 3 . 








internal combustion 


14.. 


1.0 


97.4 


Shovel and dragline , 








power. 








electric >30 yd 3 . 


6.. 


4.0 


77.4 


Scraper, tandem. 


15.. 


.8 


98.2 


Auger, coal, highwall. 


7.. 


3.8 


81.2 


Motor grader. 


16.. 


.8 


99 


Loader, track. 


8.. 


3.4 


84.6 


Drill, blasthole, with- 


17.. 


.5 


99.5 


Dozer, wheel. 








out cab. 


18.. 


.3 


99.8 


Drill , coring , truck- 


9.. 


3.3 


87.9 


Drill, blasthole, with 
cab. 








mounted . 



TABLE 2. - Major machine models in use in U.S. surface coal mines, 
by portion of machine type population, percent 



Machine type and 


Model 




Machine type and 


Model 




Machine type and 


Model 




manufacturer 






manufacturer 






manufacturer 






Dozer: 






Shovel — Con. 






Scraper: 






Caterpillar. . . 


D9 


47 




800 


4 


Caterpillar. . . 


637 


18 




D8 


17 




All 


9 




631 


12 




All 
TD25 


71 
11 




All 
3500 


8 

4 




657 
All 


7 


International . 




55 




All 
All 


12 
7 


Dragline: 


All 


8 




T524 
All 


16 


Al lis -Chalmers 




25 


Loader: 








4600 


13 




All 


5 


Caterpillar. . . 


988 


18 




4500 


8 


Blasthole drill: 








992 


19 




All 


25 


Gardner-Denver 


RDC16 


10 




All 


46 


Bucyrus-Erie. . 


88B 


9 




All 


15 




All 


13 




All 


25 


Bucyrus-Erie. . 


50R 


5 




All 


13 




2400 


13 




All 


14 


International . 


All 


6 




All 


18 


Chicago 






Hauler: 








All 


12 


Pneumatic. . . . 


650 


9 




All 


32 




All 


9 




All 


13 


Caterpillar. . . 


773 


10 


Highway truck: 








All 


13 




All 
All 


18 
12 


Ford 


FlOO 
F600 


4 
4 


Inger soil-Rand 
Grader : 


All 


11 








International . 


All 


10 




All 


28 


Caterpillar. . . 


12 


28 




All 


8 




600 


4 




16 


22 


Dart 


All 


5 




685 
All 


3 

18 




14 
All 


16 


Shovel: 


72 


Bucyrus-Erie. . 


All 


26 


General Motors 


All 


12 




All 


12 




All 


19 




All 


11 










All 


11 


International . 


All 
All 


10 
8 









NOTE. — All refers to all of the manufacturer's models in use. 



21 



Caterpillar dominates the dozer category: 
Caterpillar manufactures 71 pet of all 
dozers used in surface coal mines, 47 pet 
of which are Caterpillar model D9. 
Caterpillar also manufactures 46 pet of 
all front-end loaders used in surface 
coal mines, 18 pet of which are Caterpil- 
lar model 988. 

An obvious first step in reducing noise 
exposure is the use of operator cabs. 
Table 3 gives the percentages of machines 
that have cabs , the size of the mine in 
which the machine is operated, and 
whether the cab has any form of noise 
control (acoustical) treatment. As the 
table shows , 70 pet of all machines have 
cabs , nearly one-half of these machines 
with cabs have some kind of acoustical 
treatment, and there are more cabs in 
large mines than in small mines. In ad- 
dition, there are more acoustically 
treated cabs on newer machines than on 
older equipment; acoustically treated 
cabs came into significant use between 
1969 and 1972. 



Mobile Equipment Noise 
and Operator Exposure 

A major objective of the research was 
the calculation of the noise exposure of 
operators of various machines (see refer- 
ence 2 for detailed discussion) . For 
this calculation, independent estimates 
were made of the average working noise 
level and the time of operation (see ap- 
pendix to this paper). The average work- 
ing noise level was defined as that con- 
stant noise level that, if present during 
the entire work cycle, would result in 
the same noise exposure, or dose, result- 
ing from fluctuating noise levels that 
actually occur. Computation of the aver- 
age working noise level requires the 
typical work cycle to be divided into a 
number of events , each of which can be 
defined in terms of a typical noise level 
and percentage of the work cycle; for 
example, for a dozer, the typical work 
cycle consists of dozing, transporting, 
and backing. It is equivalent to the 
level read from a noise dosimeter 
measuring noise over one work cycle. 



TABLE 3. - Machines with cabs, by mine size, percent 



Machine 



Large 1 



Any cab 



Small^ 



All 



Acoustical cab 



Large 



Small * 



All 



58 
67 
25 
97 



Dozer, track, >150 hp 

Loader, wheel, >150 hp 

Hauler 

Truck , highway 

Shovel and dragline , internal combus- 
tion power 

Scraper , tandem 

Motor grader 

Drill , bias thole 

Scraper 

Loader, wheel, <150 hp 

Shovel and dragline, electric: 

<30 yd 3 

>30 yd 3 

Dozer , track , 150 hp 

Auger , coal , highwall 

Loader , track 

Dozer , wheel 

Drill , coring , truck mounted 

Total machine population 

^100,000-t/yr production. 2 <100,000-t/yr production 



85 
62 
60 
50 
53 
61 

91 
89 
63 
18 
65 
83 
46 



72 



57 


57 


70 


62 


86 


93 


92 


96 


83 


78 


52 


60 


64 


61 


50 


50 


27 


47 


56 


59 


80 


89 


60 


86 


45 


57 


18 


18 


41 


50 


50 


81 


50 


47 



60 



70 



35 

44 

57 

5 

33 
30 
32 
22 
24 
41 

59 
62 
36 

3 
27 
56 





37 



29 


32 


44 


44 


34 


53 


10 


7 


19 


26 


15 


26 


30 


32 


18 


22 


8 


21 


26 


35 


25 


54 


30 


58 


29 


28 


2 


2 


15 


20 


50 


55 


17 


3 



30 



34 



22 



Data on machinery noise, work cycles, 
machine usage durations, and shift 
lengths were collected during visits to 
nine mines that included both large and 
small operations , located in the Appa- 
lachian, Midwestern, and Western regions. 
Data on the noise and work cycles of over 
80 individual machines were obtained by 
direct measurement, and these data were 
supplemented by information extracted 
from interviews with mine personnel. 
Additional data were gathered from study 
reports made available by some mines , 
from the literature, and from a sample of 
records submitted by mine operators to 
one of the MSHA district offices. 

Operator exposures were evaluated on 
the basis of the most reliable data 
available. Where possible, noise data 
measured in this program were used. For 
machine types for which information was 
inadequate, estimates were based on data 
in the literature of the MSHA records. 
Daily exposure durations were taken di- 
rectly from the MSHA data. 

Table 4 shows the mean values and stan- 
dard deviations of the average working 
noise levels of the operators of various 
machines , daily operator exposure dura- 
tions, and the probabilities of operator 
over-exposure. A distinction is made to 
the extent allowed by available data be- 
tween machines with no cabs , conventional 
cabs, and acoustical cabs. 

The overexposure probabilities indicate 
the fractions of the total operator popu- 
lation that suffer overexposure according 
to the given criteria. These probabili- 
ties provide no information about how 
often (what fraction of the time) the ex- 
posure of the operator of a given 
machine exceeds the permissible limit. 
These overexposure probabilities are 
given for two criteria. The first cri- 
terion is a regulation specified in the 
Coal Mine Health and Safety Act of 1969, 
which permits exposure to 90 dBA for a 
maximum of 8 h per day and prescribes a 
reduction by a factor of 2 in the permis- 
sible daily exposure duration time for 
each 5-dBA noise level increment above 
90 dBA. The second, more stringent, 



criterion permits exposure to 85 dBA for 
a maximum of 8 h per day and again pre- 
scribes an exposure duration time reduc- 
tion by a factor of 2 for each 5-dBA in- 
crement above 85 dBA. 

As shown in table 4, operators of heavy 
track dozers without cabs are exposed to 
mean working noise levels of 103 dBA for 
a mean of 6 h per day. The last two col- 
umns of the table show that 96 pet of the 
opera-tors of dozers without cabs in sur- 
face coal mines are overexposed to noise, 
according to the current Federal regula- 
tions. If the 5-dBA reduced threshold 
criterion is adopted, 99 pet of the oper- 
ators are overexposed. The procedure for 
calculating the overexposure probability 
is given in reference 2. 

Table 4 also shows that when cabs — 
particularly cabs with noise control 
treatments — are used on any type of ma- 
chine, they decrease both working noise 
levels and the probability of overexpo- 
sure. 

For each of the various types of ma- 
chines used in U.S. surface coal mines, 
table 5 shows the total number of ma- 
chines in use (based on projections 
developed from the census data) , the 
average number of people operating each 
machine per day (based on the average 
number of daily shifts the machines are 
in use, according to the machine census), 
and the number of operators in all U.S. 
surface coal mines who may be expected to 
be overexposed (according to both the 
current criterion and the more stringent 
criterion) . The table also shows the 
percentages of the total number of opera- 
tors (approximately 56,200) who suffer 
overexposure. 

This table gives two important statis- 
tics. According to the present criteri- 
on, over 25,000 operators, or nearly 45 
pet of all mobile machine operators in 
U.S. surface coal mines, are overexposed 
to noise. With the more stringent cri- 
terion, the number of operators over- 
exposed to noise increases to over 
37,000, or more than 66 pet of the entire 
operator population. 



23 



TABLE 4. - Noise exposure of machine operators 

(Average working noise level based on reference 2 and verified mine data, 
except as otherwise noted; values are rounded to nearest 0.5 dBA) . 



Machine 



Cab 1 



Average working 
noise, level, dBA 



Mean 



Standard 
deviation 3 



Daily exposure 
duration, h 2 



Mean 



Standard 
deviation- 5 



Criterion 



Pres- 
ent 4 



5-dBA-more 
stringent 5 



Dozer, track: 
>150 hp 



<150 hp 

Dozer, wheel, 



Loader , wheel : 
>150 hp 



<150 hp , 

Loader, track., 

Hauler , 

Truck , highway , 
Scraper: 

Tandem , 



Single, 



Motor grader, 



Shovel and dragline: 
Electric: 

>30 yd 3 , 

<30 yd 3 , 

Internal combustion 

power , 

Drill: 

Bias thole , 



Coring, 
Auger. . . , 



N 
C 
A 
T 
N 
C 
A 

N 
C 
A 
T 
T 
T 
T 

N 
C 
A 
N 
C 
A 
N 
C 
A 



N 
C 
A 
T 
T 



103 

98.5 

92.6 
L 94 
e 96 

96.5 

92 

94.5 
93.5 
84.6 

'97 

L 91.5 
88.5 

e 85 

e 92 

91.5 

85 
e 96 

95.5 

91 
e 96 
'95.8 

86.5 



77.5 
86 



91 

90 

85 

83 

e 87 
e,M95 



1.5 
3.0 
4.5 
L 3.5 
e 5.0 
2.0 
6.0 

1.5 

5.0 

4.5 

e 3.0 

e 4.0 

4.5 

e 5.0 

e 7.0 

7.0 

.5 

e 5.0 

3.5 

e 5.0 

e 5.0 

'4.0 

5.0 



6.5 

4.0 

6.5 

2.0 

5.0 

3.0 

e 5.0 

e 5.0 



6.3 



5.6 



5.8 



5.1 



5.3 
5.8 

5.9 

5.6 
5.1 
5.1 
5.4 
4.1 



3.0 



2.3 



2.6 



3.1 



3.7 



1.2 
6.7 

38 

20 
7.2 
.2 
14 
48 



99 
96 
80 
83 
81 
90 
60 

93 
82 
29 
93 
69 
57 
14 

69 
67 

14 
89 
92 
69 
84 
86 
35 



6.2 
35 

64 

70 
27 

7.8 

40 

76 



'includes literature data. L From literature, 
A, acoustical cab; C, nonacoustical cab; N, no cab; T, all 



e Estimated. 
1 

2 Rounded to nearest 0.1 h. 

3 About 70 pet of all values may be expected to fall within 
low and above the mean. 

4 85 dBA permissible for 8 h daily; a reduction factor of 2 

exposure for each 5-dBA increase above 90 dBA. 

5 85 dBA permissible for 8 h daily; a reduction factor of 2 

exposure for each 5-dBA increase above 85 dBA. 



M Includes MSHA data, 
conditions . 

1 standard deviation be- 
in the permissible daily 
in the permissible daily 



24 



TABLE 5. - Projected number of machines and overexposed operators in 
U.S. surface coal mines GO 

(Projected total number of operators — 56,226) 



Machine 



Cab 1 



Number 
of ma- 
chines 2 



Operators 

per machine 

per day, 2 

average 



Overexposed operators 



Present 
criterion 3 



No. pet 



5-dBA-more-strin- 
gent criterion 4 



No. 



pet 



Dozer, track: 
>150 hp 



<150 hp 

Dozer, wheel, 



Loader , wheel : 
>150 hp 



<150 hp , 

Loader, track., 

Hauler , 

Truck , highway , 
Scraper: 

Tandem , 



Single, 



Motor grader, 



Shovel and dragline : 
Electric: 

>30 yd 3 , 

<30 yd 3 

Internal combustion 
Drill: 

Blasthole 



power 



Coring , 

Auger , 

Total or average, 



N 
C 
A 
T 
N 
C 
A 

N 
C 
A 
T 
T 
T 
T 

N 
C 
A 
N 
C 
A 
N 
C 
A 



T 
T 

T 

N 
C 
A 
T 
T 



4,551 

2,648 

3,447 

584 

24 

32 

71 

2,149 
1,661 
2,991 
1,033 
411 
5,620 
2,939 

462 
393 
310 
486 
252 
197 
549 
411 
450 



234 

334 

3,273 

1,316 
721 
558 
109 
323 



1.56 
1.34 
1.92 

1.36 

1.30 
1.13 
1.52 
1.25 

1.47 

1.33 

1.19 



3.14 
2.20 
1.45 

1.20 
1.75 
1.75 
1.47 
1.53 



6,816 


12.1 


3,635 


6.5 


2,635 


4.7 


446 


.8 


25 


<.l 


40 


.1 


44 


.1 


2,163 


3.8 


1,265 


2.2 


240 


.4 


1,061 


1.9 


172 


.3 


1,965 


3.5 


95 


.2 


299 


.5 


237 


.4 





<.l 


446 


.8 


238 


.4 


97 


.2 


405 


.7 


313 


.6 


59 


.1 


9 


<.l 


49 


.1 


1,803 


3.2 


316 


.6 


91 


.2 


2 


<.l 


22 


<.l 


237 


.4 



7,028 

3,966 

4,302 

649 

37 

55 

82 

2,718 
1,852 
1,179 
1,249 

320 
4,869 

514 

469 
387 
64 
575 
308 
181 
549 
421 
187 



46 

257 

3,037 

1,105 

341 

76 

64 

376 



NAp 



38,539 



1.46 



25,225 



44.9 



37,263 



12.6 

7.1 

7.7 

1.2 

.1 

.1 

.1 

4.8 
3.3 
2.1 
2.2 

.6 
8.7 

.9 

.8 
.7 
.1 

1.0 
.5 
.3 

1.0 
.7 
.3 



.1 
.5 

5.4 

2.0 
.6 
.1 
.1 
.7 



66.3 



NAp Not applicable. 
A, acoustical cab; C, nonacoustical cab; N, no cab; T, all conditions. 



2 Reference 1. 

3 85 dBA permissible 
posure for each 5-dBA 

4 85 dBA permissible 
posure for each 5-dBA 



for 8 h daily; a reduction factor of 2 in permissible daily ex- 
increase above 90 dBA. 

for 8 h daily; a reduction factor of 2 in permissible daily ex- 
increase above 85 dBA. 



25 



Table 5 also shows that all types of 
dozers together are responsible for over- 
exposure of over 13,600, or over 24 pet 
of all surface mine operators; that is, 
dozers contribute more than 50 pet of all 
noise overexposures. Loaders, in turn, 
overexpose more than 4,900 operators, or 
8.6 pet of all surface mine operators, 
and they account for slightly less than 
19 pet of all noise overexposures. 

The next most significant categories 
lag far behind dozers and loaders. They 
are haulers, which overexpose nearly 
2,000 operators (3.5 pet of all opera- 
tors, 8 pet of all overexposed operators) 
and diesel-powered shovels and draglines, 
which overexpose about 1,800 operators 



(3.2 pet of all operators, 6 pet of all 
overexposed operators). 

Two facts should be noted, because they 
have a bearing on the over-exposures 
shown in table 5. Most of the overexpo- 
sure associated with haulers results from 
haulers being operated with open windows ; 
haulers whose noise is measured with 
their windows closed rarely present a 
noise overexposure problem. Similarly, 
the data base for shovels and draglines 
powered by internal combustion engines is 
biased toward older models because newer 
models tend to be much quieter. As a re- 
sult, the overexposures indicated for 
haulers and for shovels and draglines may 
be overestimated. 



NOISE CONTROLS 



The extent of operator overexposure, 
the types of mobile machines responsible 
for that overexposure, and the results of 
the first study, were published in a Bu- 
reau report ( 1_) . As a result of this 
study, the Bureau sponsored research to 
prove cost-effective retrofit noise con- 
trol technology for surface mobile equip- 
ment. Because bulldozers and front-end 
loaders are responsible for approximately 
two-thirds of the noise overexposure 
problem, they were chosen as representa- 
tive machines for this project. Descrip- 
tion of this project is prefaced by a 
general discussion of noise control 
techniques — the tools of noise control. 

Major Sources and Paths 

In general, the noise from any one 
source reaches the ear via several paths , 
both directly, by airborne paths, and in- 
directly, by reflections from various 
surfaces. In addition, sound in the form 
of vibration may travel along or through 
structures. 

In diesel-powered mining equipment , the 
engine is generally a major source of 
noise. Engine noise may come from the 
exhaust, the intake, and the casing (that 
is, the block and accessories attached to 
it) — as well as the cooling fan — often a 



significant noise source. The trans- 
mission, drive train, and hydraulic sys- 
tem also tend to be significant noise 
sources. 

Noise radiated from the various sources 
may reach the operator by propagating 
through the air, directly or by reflec- 
tions. In addition, vibrations produced 
by the engine and other mechanical com- 
ponents , as well as structural vibrations 
caused by sounds , tend to propagate along 
machine structures , thus causing these 
structures to radiate sound. 

The relative importance of the various 
noise sources and paths differs for dif- 
ferent machine types and models. How- 
ever, one fact is basic for all machines: 
Just as repair of small holes in a leak- 
ing roof is useless if large holes are 
left open, reducing the noise of lesser 
sources and paths has practically no ef- 
fect on a worker's exposure unless the 
contributions from the major sources and 
paths are reduced. In addition, it does 
not usually make sense to spend the money 
to quiet dominant sources and paths to 
the point where their contributions are 
far below those of the lesser sources and 
paths. Overquieting is both impractical 
and costly. 



26 



Noise Reduction of Diesel-Powered 
Equipment 

In general, the noise exposure of an 
operator of a given machine may be re- 
duced by blocking the paths of sound be- 
tween the important noise sources and the 
miner. Usually, for both practical and 
economical reasons , the primary noise 
sources cannot be modified or replaced 
with quieter ones (except relatively 
early in the development of new ma- 
chines). Generally then, the first solu- 
tion to a problem of mine machine noise 
is blocking the noise paths , both air- 
borne and structureborne. 

Cabs generally are the most cost effi- 
cient way to obstruct the radiation of 
sound from such sources as engines or 
transmissions. The effectiveness of such 
an enclosure increases with the mass of 
its walls, and effectiveness is greater 
if the cab is lined with some kind of 
acoustically absorptive material. If a 
full cab installation would present prob- 
lems of cooling or access, partial cabs 
or barriers may be used. They tend to be 
considerably less effective in noise re- 
duction than full cabs because they do 
not provide the operator with noise at- 
tenuation from all directions , which in- 
creases the operator exposure to both 
direct and reflected noise. In a partial 
cab, the noise the operator hears is not 
passing through it, but traveling around 
it. As a result, increasing the mass of 



the barrier (an effective way to reduce 
noise heard in full cabs), usually re- 
sults in little noise reduction in par- 
tial cabs. 

Mufflers obstruct the propagation of 
sound out of pipes or ducts, primarily by 
reflecting some of the sound back toward 
the source so that the reflected pressure 
waves almost cancel out the outgoing 
waves. It is important to match engine 
exhaust mufflers to the engine, so that 
they will be effective acoustically, yet 
not produce excessive backpressure. Muf- 
flers are commericially available for 
almost all pieces of equipment used in 
U.S. surface mines. 

One of the most overlooked ways to re- 
duce noise levels is machine maintenance. 
Table 4 shows a number of machine cate- 
gories , such as highway trucks , with 
standard deviations of 4 dBA or more. 
This is a significantly large variation 
between the noise of one machine and 
another in the same category. There 
could be several reasons , of course , but 
experience has shown that a major contri- 
bution is the state of repair of the in- 
dividual machine. Are the seals tight? 
Are all windows in place? Is the air 
conditioner working so the operator will 
not need to open the windows (letting in 
air and also noise)? Are the floormats 
in place? Proper maintenance of the 
machine is a must for successful noise 
control. 



RESULTS OF RETROFIT NOISE CONTROL PROGRAM 



The Bureau's surface mining noise con- 
trol research program has concentrated on 
retrofit acoustical cab treatments for 
bulldozers and front-end loaders. Models 
of both machine types were selected for 
these treatments , based on their overall 
popularity in the surface mining indus- 
try. The retrofitted dozers and front- 
end loaders were field tested in surface 
coal mines for a period of about 1.5 yr. 
In general, mine operators were satisfied 
with the durability and effectiveness of 
the noise control treatments. 



Detailed fabrication manuals , contain- 
ing photographs and illustrations that 
show how the noise control treatments 
were installed, have been prepared for 
both bulldozers (4) and front-end load- 
ers (6). Numerous Bureau-sponsored work- 
shops were held throughout the country to 
provide equipment users with a closer 
look at the retrofit process. Most of 
the workshop attendees found them bene- 
ficial, and many equipment users have 
since applied the noise control treat- 
ments to their own bulldozers and front- 
end loaders. 



27 



Bulldozers 

A breakdown of the 1977 bulldozer pop- 
ulation in U.S. surface mines (table 6) 
shows that the Caterpillar model D9 was 
by far the most popular model, comprising 
47 pet of the population (1_) . For this 
reason, the Bureau treated two different 
varieties of D9 dozers , one with only a 
ROPS-FOPS structure 4 and one with a com- 
plete (but not acoustical) cab. To show 
that retrofit noise control treatments 
could also be applied successfully to 
another manufacturer's bulldozer, an In- 
ternational Harvester model TD-25C ma- 
chine (ROPS-FOPS only) was also treated. 

TABLE 6. - Total bulldozer population 
by machine model, U.S. surface 
mines, 1977, pet 

Caterpillar D9 47 

Caterpillar D8 17 

International Harvester TD-25.... 11 

All others 25 

Although the design details of the 
three machines were somewhat different, 
the same four basic treatments were used: 
(1) installing a muffler on the diesel 
engine exhaust, (2) sealing numerous 
holes in the floor and dashboard of the 
operator's station, (3) adding sound- 
absorbing materials under the ROPS-FOPS 

4 ROPS, rollover protective structure; 
FOPS, falling object protective struc- 
ture. Modifications to ROPS-FOPS must be 
approved and performed by qualified peo- 
ple. Also, flame resistant materials 
should be used. 



structure and under the cover of the 
hydraulic tank, and (4) installing 
vibration-isolation materials between the 
engine and dashboard. In addition, wind- 
shields were installed on the two dozers 
that originally contained only ROPS-FOPS 
structures. These windshields were ex- 
tremely important because they blocked 
the direct path between the diesel engine 
(the largest single noise source on the 
machines) and the dozer operators. Seals 
were also installed around the doors of 
the cab-equipped D9 dozer. 

Table 7 summarizes the noise reductions 
achieved through the dozer retrofit 
treatments, the cost of the acoustical 
materials and hardware, and the labor 
hours needed to install them. Note that 
the operator noise levels after treatment 
(89-94 dBA) were low enough to permit 6 
to 8 h of daily operating time without 
violating Federal noise regulations; 
before treatment , only 1 to 2 h of op- 
erating time were allowed. The effects 
of the individual treatments on the three 
machines in table 7 are described. 

Caterpillar D-9G With ROPS-Fops Only 

Figure 1 is a photograph of the treated 
dozer, and figure 2 shows the seven major 
components of the retrofit noise control 
package. Diagnostic tests of the un- 
treated dozer indicated that the wind- 
shield would be the single most effective 
noise control treatment, followed by the 
ROPS-FOPS canopy absorption and the en- 
gine exhaust muffler; therefore, these 
three treatments were installed first. 



TABLE 7. - Summary of results of bulldozer retrofit noise control treatments 





Caterpillar D-9G 


International Harvester TD- 




ROPS-FOPS only 


With cab 


25C, ROPS-FOPS only 


Operator noise level, dBA: 


105 

94 

11 

$825 

106 


1 99-2 100 

2 89- 1 9l 

9-11 

$725 

88 


102 




91 




11 




$912 
80 





Cab doors open. 2 Cab doors closed. 



28 




FIGURE 1. - Caterpillar D-9G bulldozer (ROPS-FOPS only) with retrofit noise control treatments. 



Figure 3 shows how the operator noise 
level decreased as each of the seven 
treatments was added. The overall noise 
reduction was 11 dBA, but the reduction 
obtained through one treatment depended 
on the presence of the previous treat- 
ments. For example, the windshield alone 
would have reduced the noise by about 4 
dBA, the canopy absorption alone would 
have reduced the noise by about 3 dBA, 
and the exhaust muffler alone would have 
reduced the noise by about 1.5 dBA. The 
remaining treatments would have had a 
negligible effect on operator noise if 
the windshield, canopy absorption, and 
muffler had not been installed; there- 
fore, these three treatments were by far 
the most important components of the 
retrofit package. Table 8 summarizes the 
material costs and labor hours associated 
with each component of the package. 



Caterpillar D-9G With Cab 

Figure 4 shows the six major components 
of the retrofit noise control package 
applied to the cab-equipped D-9G dozer. 
This machine already had a relatively new 
muffler, so none was installed; however, 
this would ordinarily have been included 
in the package. Since a windshield was 
already a part of the cab, the simpler 
cab wall seal treatment replaced the 
windshield treatment required for the D- 
9G dozer with ROPS FOPS only. The in- 
terior walls of the cab were treated with 
the same sound-absorbing materials as the 
underside of the canopy. Note in table 7 
that the operator noise level in the un- 
treated cab was higher when the doors 
were closed than when they were open; 
this occurred because the untreated doors 
tended to rattle in their sockets when 



29 



FOPS canopy absorption 



Seat seals 



Muffler 



Hydraulic valve 
cover and tank 
seal 



Dashboard seals V_ 
and vibration 
isolation 




Floormat and seals 



FIGURE 2. - Noise control treatments installed 
on Caterpillar D-9G bulldozer (ROPS-FOPS only). 





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TREATMENT 















FIGURE 3. - Step-by-step noise reduction 
treatments of Caterpillar D-9G bulldozer. 



they were closed. After the door seals 

were added, however, the operator noise 

exposure level was lower when the doors 
were closed. 

Figure 5 shows how the operator noise 
level decreased as the treatments were 
added (11-dBA total reduction, cab doors 
closed) . As with the D-9G dozer with 



R0PS-F0PS only, the noise reduction at 
each stage of treatment depended on the 
presence of the previously installed 
treatments . Table 9 summarizes the ma- 
terial costs and labor hours associated 
with each component of the package. Note 
that the cost of a muffler (although not 
needed on this particular machine) is al- 
so included in table 9. 



TABLE 8. - Summary of material and labor costs for noise control package on 
Caterpillar D9G dozer with R0PS-F0PS only 





1978 ma- 
terial 
costs 2 


Labor 1 


Treatment 


1978 ma- 
terial 
costs 2 


Labor ' 


Treatment 


Weld- 
er 


Me- 
chanic 


Weld- 
er 


Me- 
chanic 




$275 

115 
190 

25 
80 


29 

4 
NAp 

1 

NAp 


16 

10 
2 

4 
8 




$55 

80 
5 


NAp 

4 
2 


8 


FOPS canopy 
Exhaust muffler.... 


Hydraulic valve 
cover and tank 


16 


Dashboard seals-vi- 


Miscellaneous items 
Total 


2 


bration isolation. 


825 


40 


66 


Floormat and seals. 







NAp Not applicable. 'Estimated worker-hours. 2 Approximate. 



30 



TABLE 9. - Summary of material and labor costs for noise control package on 
Caterpillar D9G dozer with cab 





1978 ma- 
terial 
costs 2 


Labor 1 


Treatment 


1978 ma- 
terial 
costs 2 


Labor ' 


Treatment 


Weld- 
er 


Me- 
chanic 


Weld- 
er 


Me- 
chanic 


FOPS canopy 


$150 
75 
80 

105 

95 


4 
2 
2 

4 

NAp 


8 

22 

8 

12 

16 


Dashboard seals-vi- 
bration isolation. 
Miscellaneous items 


$25 
5 


2 
2 


4 
2 


Floormat and seals. 
Seat and hydraulic 


535 
190 


16 
NAp 


72 
2 


Sound absorption on 


Grand total... 


725 


16 


74 



NAp Not applicable. 'Estimated worker-hours. 



Approximate, 



International Harvester TD-25C With 
ROPS-FOPS Only 

Figure 6 shows the major components of 
the retrofit noise control package for 
the TD-25C dozer. Since the manufacturer 
had already installed an exhaust muffler 
on this machine, none was needed; how- 
ever, a muffler would have to be in- 
stalled if none were present. Figure 7 
shows how the operator noise level de- 
creased as the treatments were added (11- 
dBA total reduction) , and table 10 sum- 
marizes the material costs and labor 
hours associated with each component of 
the package. 

Comparison of tables 10 and 8 shows 
that the cost of the windshield was the 



FOPS canopy absorption 



Sound absorption on 
cab interior 



Seat and hydraulic 
valve seals 



Dashboard seals and 
vibration isolation 




Floormat and seals 



Cab wall seals 



FIGURE 4. - Noise control treatments on cab- 
equipped Caterpillar D-9G bulldozer. 



biggest difference between the retrofit 
packages for the International Harvester 
TD-25C and the Caterpillar D-9G with 
ROPS-FOPS structures. More materials and 
labor were needed for the TD-25C wind- 
shield because it was larger and more 
difficult to fabricate, but it provided 
more noise reduction (5 versus 4 dBA) 
than the D-9G windshield. As with the 



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FIGURE 5. - Step-by-step noise reduction treat- 
ments of cab equipped Caterpillar D-9G bulldozer. 
Note that because this dozer was already equipped 
with a relatively new muffler, the treatments do 
not include installation of a new muffler. How- 
ever, an effective muffler should be part of any 
noise control treatment if the machine has no muf- 
fler as the existing muffler is badly rusted. 



31 



TABLE 10. - Summary of material and labor costs for noise control package on 
International Harvester TD-25C bulldozer with ROPS-FOPS only 





1978 ma- 
terial 
costs 2 


Labor 1 


Treatment 


1978 ma- 
terial 
costs 2 


Labor ' 


Treatment 


Weld- 
er 


Me- 
chanic 


Weld- 
er 


Me- 
chanic 




$537 

182 
43 


8 

1 

NAp 


53 

8 
2 


Seat seals , hydrau- 
Total 


$150 


0.5 




FOPS canopy 


7.5 




912 
400 


9.5 

NAp 


70.5 


Option: Dashboard 






7 



NAp Not applicable. 'Estimated worker-hours. 2 Approximate. 



other two bulldozers, the noise reduc- 
tions achieved with individual treatments 
on the TD-25C depended on the presence of 
the other treatments. The windshield and 
the ROPS-FOPS canopy absorption were the 
two most important treatments; the others 
would have been ineffective without them. 
The dashboard barrier was considered to 
be an optional treatment because its cost 
was high compared to the noise reduction 
resulting from its installation. 

Front End Loader (5-6) 



The front-end loader ranked second to 
the bulldozer as a "noise offender" 
in the surface mining industry (1_) . 
Although about 40 pet of the loaders 



identified during the 1977 census were 
equipped with factory-designed acoustical 
cabs ; the remaining 60 pet required some 
type of retrofit noise control treatment. 
The Bureau chose two of the most popular 
loader models for the retrofit program — a 
Caterpillar 988 and an International Har- 
vester H-400 B. Both machines had non- 
acoustical operator cabs ; this made the 
retrofit treatments easier to install 
than if they had equipped only with ROPS- 
FOPS structures. 

The treatments themselves were simi- 
lar to those installed on the cab 
equipped bulldozer: (1) exhaust muf- 
flers, (2) seals around openings in the 
cab walls, doors, seats, and floors, and 



ROPS canopy absorption 

Windshield 



Dashboard barrier 




Hydraulic box seals 



FIGURE 6. - Noise control treatments installed 
on International Harvester TD-25C bulldozer. 



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TREATMENT 



FIGURE 7. - Step-by-step noise reduction treat- 
ments of International Harvester TD-25C bulldozer. 



32 



(3) sound-absorbing materials on all the 
interior cab surfaces, including the can- 
opies. Table 11 summarizes the noise re- 
ductions, material costs, and labor hours 
associated with the retrofit treatments. 
As with the cab-equipped bulldozer, the 
noise levels in the untreated loader cabs 
were the same or greater when the doors 
were closed than when they were open, due 
to the doors rattling in their sockets. 
After treatment, however, the loader cabs 
were quieter when the doors were closed. 
Note also that the costs of installing 
exhaust mufflers are not included in 



table 11 because the loaders already had 
mufflers. If mufflers had not been pres- 
ent, they would have been essential com- 
ponents of the noise control packages. 

Figures 8 and 9 and table 12 describe 
the retrofit package on the Caterpillar 
988 loader; figures 10 and 11 and table 
13 do the same for the International H- 
400 B. As with the bulldozer retrofit 
packages, the effectiveness of each suc- 
cessive noise control treatment depended 
on the presence of previous treatments. 



TABLE 11. - Summary of results of front-end loader retrofit noise 
control treatments 





Caterpillar 988 


Int( 


srnational Harvester H-400 B 


Operator noise level, 


dBA: 




^-^Ol 

2 90- 1 91 

2 11 

$410 

29 




1,295 




2 83- 1 87 






. .dBA.. 


2 12 


1978 material costs . 






$580 
19 





Cab doors open. 2 Cab doors closed. 3 Not including exhaust muffler. 



TABLE 12. - Summary of material and labor costs for noise control package on 
Caterpillar 988 front-end loader 



Treatment 


1978 ma- 
terial 
costs 2 


Labor , 1 
mechanic 


Treatment 


1978 ma- 
terial 
costs 2 


Labor, ' 
mechanic 


Canopy and rear cab 
wall sound absorption 


$58 

200 
39 
50 


7 

8 
3 
6 


Additional. sound ab- 
sorption on cab 


$63 


5 




Total 


410 


29 









'Estimated worker-hours, 



2 Approximate, 



TABLE 13. - Summary of material and labor costs for noise control package on 
International Harvester H-400B front-end loader 



Treatment 


1978 ma- 
terial 
costs 2 


Labor, ' 
mechanic 


Treatment 


1978 ma- 
terial 
costs 2 


Labor, ' 
mechanic 




$138 
393 


8 

8 




$49 


3 


Cab sound absorption.. 




580 


19 



Estimated worker-hours, 



■Approximate. 



33 



Canopy and rear cab wal 
sound absorption \^ 



Floormat \q 

Additional sound \k[U 
absorption on cab / 
interior 



Pedestal seals 



Cob wal! seals 




FIGURE 8. • Noise control treatments installed 
on Caterpillar 988 front-end loader. 



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TREATMENT 

FIGURE 9. - Step-by-step noise reduction treat- 
ments of Caterpillar 988 front-end loader. 



CONCLUSIONS 



The noise exposure of U.S. 
miners was evaluated. A 1977 
cated that over 25,000 mobile 
erators (nearly 45 pet of 
mately 56,200 operators) were 
to noise according to the cr 
cified in the Federal Coal 
and Safety Act of 1969 and 
amendments of 197 7. 5 



surface coal 
study indi- 
machine op- 

the approxi- 
overexposed 

iterion spe- 
Mine Health 
subsequent 



D It is estimated that for all surface 
mines (coal and metal and nonmetal) that 
over 50,000 mobile machine operators are 
overexposed to noise. 



Heavy track dozers were the largest 
contributors, responsible for 54 pet of 
the overexposure , and rubber-tired front- 
end loaders were second in importance , 
contributing to 19 pet of the overexpo- 
sure. On the basis of these results, the 
Bureau selected bulldozers and front-end 
loaders for which to develop and prove 
cost-effective retrofit noise control 
techniques. Durable cost-effective noise 
control techniques were proven and well 

documented, which, in general, provide 
over 10 dBA of noise reductions at typi- 
cal costs of $1,000 in materials and 100 
h of labor. 



34 



Canopy absorption 



Cab wall sea: 



Cab door seals 



Floormat 



Cab wall absorption 






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FIGURE 10.- Noise control treatments installed 
on International Harvester H-400 B front-end loader. 



TREATMENT 

FIGURE 11. - Step-by-step noise reduction treat- 
ments of International Harvester. 



35 



REFERENCES 



1. Ungar, E. E. A Census of Mobile 
Machines Used in U.S. Surface Coal Mines 
(contract J0166057, Bolt Beranek and 
Newman, Inc. and Woodward Associates, 
Inc.). BuMines OFR 77-78, 1977, 174 pp.; 
NTIS PB 284 112. 

2. Ungar, E. E., D. W. Andersen, and 
M. N. Rubin. The Noise of Mobile Ma- 
chines Used in Surface Coal Mines : 
Operator Exposure, Source Diagnosis, Po- 
tential Noise Control Treatments (Bolt 
Beranek and Newman, Inc.). BuMines OFR 
98-79, 1978, 117 pp.; NTIS PB 299 538. 

3. Madden, R. , and M. Rubin. Noise 
Control of Large Truck Dozers Used in 
Surface Mining. Final report on BuMines 
contract J0177049 with Bolt Beranek and 
Newman, Inc., April 1983, 58 pp.; avail- 
able upon request from R. C. Bartholomae, 
Pittsburgh Research Center, Bureau of 
Mines, Pittsburgh, PA. 



4. Bolt Beranek and Newman, Inc. 
Bulldozer Noise Control Manual. Report 
under BuMines contract J0177049, May 
1980, 270 pp.; available upon request 
from R. C. Bartholomae, Pittsburgh Re- 
search Center, Bureau of Mines, Pitts- 
burgh, PA. 

5. Dixon, N. R. , and A. R. Thompson. 
Noise Control of Rubber-Tired Front-End 
Loaders Used in Surface Mines. Final re- 
port on BuMines contract J0395028 with 
Bolt Beranek and Newman, Inc., June 1983, 
52 pp.; available upon request from R. C. 
Bartholomae, Pittsburgh Research Center, 
Bureau of Mines, Pittsburgh, PA. 

6. Bolt Beranek and Newman, Inc. 
Loader Noise Control Manual. Report un- 
der BuMines contract J0395028, June 1981, 
130 pp.; available upon request from 
R. C. Bartholomae, Pittsburgh Research 
Center, Bureau of Mines, Pittsburgh, PA. 



36 



APPENDIX A—CALCULATION OF AVERAGE WORKING NOISE LEVEL 



The average working noise level is a 
useful concept for characterizing the 
noise exposure contribution of a given 
machine that produces nonconstant noise 
levels. The average working noise level 
is defined as the constant noise level 
that results in the same noise dose as 
the actual nonconstant noise levels , for 
the period the machine is operating. 

For example, consider a machine that 
subjects its operator to a 90-dBA noise 
level while it idles and to a 95-dBA lev- 
el while it is used at full power; assume 
also that the machine operates at idle 
for 30 pet of the time it is in use and 
at full power 70 pet of the time. Note 
from table A-l that the permissible expo- 
sure duration for 90 dBA is 8 h and for 
95 dBA, 4 h. Assuming a total of 7 h of 
use per day (7 x 0.30 = 2.10 h at 90 dBA 
and 7 x 0.70 = 4.90 h at 95 dBA), a total 
noise dose of 2.10/8 + 4.90/4 = 1.487 
is obtained. The average working noise 
level in this case is that noise level 



level producing a noise dose of 1.487, 
if it is continuous for 7 h. 

The permitted exposure duration, T, in 
hours is related to the noise dose, D, 
and actual exposure duration, C, in hours 
as 

T = C/D. 

Thus, here the permitted duration, T, is 
7/1.487 = 4.71 h. From the values indi- 
cated in table A-l , one may observe that 
the average working noise level is be- 
tween 92 and 95 dBA. The exact value of 
93.8 dBA can be calculated from the equa- 
tion in the note to table A-l. 

The assumed value of the daily use time 
(taken above as 7 h) does not affect the 
value of the average working noise level. 
The effects of the assumed values of the 
daily use time cancel , because the same 
number is used in the dose evaluation 
calculation and in the determination of 
the corresponding permitted durations. 



TABLE A-l. - Permissible noise exposures 



Duration of exposure 
per day, h 



Noise 
level, dBA 

90 
92 
95 

97 
100 



Duration 


of 


exposure 


1 


.5... 


per 


day 


, h 


1 













.5... 




















Noise 
level, dBA 

102 
105 
110 
115 



NOTE. — Noise levels are measured with a sound level meter set to slow 
response. Exposure to continuous levels above 115 dBA is not permitted 
by law. Values between those tabulated may be obtained from log T = 
6.322 - 0.602 SL, where T denotes the exposure duration, h, and SL is the 
sound level, dBA. 



37 



A REDUCED-NOISE AUGER MINER CUTTING HEAD 
By William W. Aljoe 1 and Mark R. Pettitt 2 



ABSTRACT 



After extensive laboratory and in-mine 
tests, a cost-effective, mineworthy, 
reduced-noise auger miner cutting head 
was designed, fabricated, and field test- 
ed by Wyle Laboratories , under Bureau of 
Mines contract HO188065. Compared with 
standard auger cutting heads, the new 
heads reduced noise by 10 dBA at the 



jacksetter's position and 6 dBA at the 
operator's position. The reduced-noise 
heads were able to cut and load coal as 
effectively as the standard heads and are 
now being used successfully in several 
underground mines. Mine shops can easily 
modify the standard auger cutting heads 
to produce the reduced-noise design. 



INTRODUCTION 



Auger-type continuous miners are de- 
signed to extract coal from thin seams , 
approximately 26 to 50 in. in height. 
Figure 1 shows one model of auger miner, 
the Fairchild (Wilcox) Mark 21. 3 The two 
rotating augers at the front of the miner 
cut the coal and move it to the chain 
conveyor at the center of the machine. 
The conveyor carries the coal to the rear 
of the machine and dumps it onto a bridge 
conveyor system. The bridge conveyor 
connects with a panel conveyor (panline) , 
which removes the coal from the face 
area. 

Figure 2 describes the cutting pattern 
of the auger-type continuous miner. Note 
in figure 2 that the "anchor jack" is 



placed very close to the face before each 
arc-shaped cut is made. On the Mark 21 
miner in figure 1, the hydraulic anchor 
jacks are emplaced remotely by the ma- 
chine operator. However, on the older 
Mark 20 auger miner shown in figure 2, 
the anchor jacks are simple mechanical 
posts , emplaced manually by workers 
called jacksetters. In addition, both 
models of auger miners require the pres- 
ence of timbermen and/or cleanup person- 
nel in the immediate face area. Because 
of their close proximity to the cutting 
heads, the jacksetters, timbermen, and 
cleanup personnel on auger mining sec- 
tions are exposed to more noise and dust 
than almost all other workers in under- 
ground coal mines. 



APPROACH TO AUGER MINER NOISE CONTROL 



Noise levels in a typical auger mining 
section during coal cutting are approxi- 
mately 106 to 108 dBA at the jacksetter's 
position and 102 dBA at the operator's 
position. 4 To comply with Federal noise 

fining engineer, Pittsburgh Research 
Center, Bureau of Mines, Pittsburgh, PA. 

o 

^Senior research engineer, Wyle Labora- 
tories, Huntsville, AL. 

^Reference to specific products does 
not imply endorsement by the Bureau of 
Mines. 

4 Bobick, T. G., and D. A. Giardino. 
Noise Environment of the Underground Coal 
Mines. MESA IR 1034, 1976, 26 pp. 



regulations, operating time per shift 
would have to be limited to less than 45 
min for the jacksetter and about 1-1/2 h 
for the operator. For this reason, MSHA 
and the Bureau of Mines have investigated 
ways to reduce the noise associated with 
auger mining systems. 

The three major noise sources on stan- 
dard auger miners are the cutting heads , 
the chain conveyor, and the hydraulic 
motors, in decreasing order of impor- 
tance. The cutting process is by far the 
dominant noise source at the jacksetter's 
position; cutting noise is approximately 
equal to the sum of conveyor noise and 



38 




FIGURE 1. - Fairchild (Wilcox) Mark 21 auger miner. 




Miner pivots on extended right 
pivot jack as it swings to right 
making cut /. Retracted left 
pivot jack swings forward toward 
cut 2 pivot point. 




Pivoting on extended left pivot 
jack, miner swings to left through 
cut 2. Retracted right pivot jack 
advances toward cut J pivot point. 




Again pivoting on extended right 
pivot jack, miner swings right, 
making cut 3. Retracted left 
pivot jack moves ahead toward 
cut 4 pivot point. 



FIGURE 2. - Cutting sequence of auger-type continuous miners. 



39 



motor noise at the machine operator's 
position. By installing noise control 
treatments on the chain conveyor and mo- 
tors of a Mark 20 auger miner, MSHA was 
able to reduce the noise level at the 
operator's position by 5 dBA (from 102 to 
97 dBA). 5 Because cutting head noise was 
not addressed in this study, the Bureau 
contracted (HO188065) with Wyle Labora- 
tories for development of a reduced-noise 
auger miner cutting head. This paper 
summarizes the research conducted under 
contract H0188065.6 



Four major steps were involved in the 
development of the reduced-noise auger 
cutting head: (1) defining the forces 
that excite the cutting head (i.e., 
coal-cutting forces); (2) defining the 
vibrational response characteristics of 
the standard head; (3) developing new 
cutting head designs based on steps 1 and 
2; and (4) building several new cutting 
heads and testing them in an underground 
coal mine to verify their noise-reducing 
capabilities and overall operational per- 
formance compared to standard auger cut- 
ting heads. 



CUTTING HEAD EXCITATION (COAL CUTTING FORCES) 



Basic research in coal-cutting mechan- 
ics 7 showed that coal (and rock) resists 
the advance of a cutting bit in a manner 
analogous to a spring. The cutting force 
of the bit increases until the tensile 
stress in the coal initiates a localized 
brittle fracture. This fracture propa- 
gates out from the point of force appli- 
cation for a small distance. The bit 
continues to advance through fractured 
coal, meeting relatively little resist- 
ance until it again contacts unfractured 
coal, when the process is repeated. 

Figure 3 is a graph of coal cutting 
force versus time; it clearly shows the 
impulsive nature of the coal cutting 
process. Note that the initial impact 
force of the bit against the coal is no 
higher nor longer lasting than the subse- 
quent fracture events. Extensive labora- 
tory experiments showed that both the 
stress required to initiate fracture and 
the characteristic distance of fracture 
propagation are relatively independent of 

5 Giardino, D. A., T. G. Bobick, and 
L. C. Marraccini . Noise Control of an 
Underground Continuous Miner, Auger-Type. 
MESA IR 1056, 1977, 57 pp. 

^Ongoing BuMines contract; for inf. 
contact W. W. Aljoe, TPO, BuMines, Pitts- 
burgh, PA. 



cutting bit velocity. Thus, the peak 
force that initiates coal fracture and 
the total number of fracture events 
that occur in a given length of cut 
are statistical constants for a given 
type of coal, depth of cut, and bit 
configuration. 

Figure 4 is a plot of cutting force 
(power spectral density) versus frequen- 
cy, taken from the force-time history of 
figure 3. Note that the cutting force is 
relatively flat until the "cutoff fre- 
quency," after which it declines with 
frequency at a rate that is inversely 
proportional to the square of the fre- 
quency. The cutoff frequency is pri- 
marily a function of the coal type and 
the cutting bit velocity — the faster the 
cut, the higher the cutoff frequency. 
The standard auger miner produced a cut- 
off frequency of around 100 to 200 Hz; 
this was an important factor affecting 
the design of the reduced-noise cutting 
head. 

7 Becker, R. S., and G. R. Anderson. An 
Investigation of the Mechanics and Noise 
Associated With Coal Cutting. Wyle Lab. 
Rep. TM 81-13, Nov. 1981, 107 pp.; avail- 
able upon request from M. R. Petti tt, 
Wyle Lab., Huntsville, AL. 



40 



,000 



-o 500 - 



O 

a: 
o 







500 



1 1 1 
Peak force = 633 lb 


Mean cutting force = 309 1 b 

V Si 


i i i 


i i i i 







25 50 75 100 125 

TIME,ms 

FIGURE 3. - Coal-cutting force versus time. 



50 



175 200 




20 40 



100 200 400 1,000 2,000 4,000 10,000 

FREQUENCY, Hz 

FIGURE 4. - Coal-cutting force (power spectral density) versus frequency. 



41 



CUTTING HEAD VIBRATION CHARACTERISTICS 



The standard auger cutting head (fig. 
5) consists of three major structural 
elements — a cylindrical core surrounded 
by two spiral, cantilevered plates called 
helixes. (The "head casting" at the 
front of the auger contains both a cylin- 
drical core portion and two cantilevered 
helix portions.) When a simulated cut- 
ting force was applied to the cutting 
bits, it was found that the vibrational 
response of the helixes was much greater 
than the response of the core. There- 
fore, control of helix vibration was 
identified as the key to controlling cut- 
ting head noise. 

The mass, stiffness, and damping char- 
acteristics of the helix determine its 



Cutting helix 
Conveying helix ,. 







FIGURE 5. - Standard auger miner cutting head. 



I IOi — m — i — r~ i — n — r~m — i — i i i i — r~\ — rn — i i i i — r~\ i i i i i i i i i i 



In mine, 109.81 dB r -| 




50l — i i i i i 



I I I I I I I I I I 1 I I L 



J 1 I I L 



J I I I I L 



2.5 25 50 100 200 400 800 1,600 3,150 6,300 12,600 

16 L 31.5 L63 L 125 k250 C 500 C 1,000 C 2,000 l>,000 C8 000 C 16,000 

20 ^si 40 V <L 80 \^ 160 \I^3I6 ^< 630 ^H^ 1,250^4 2,500 ^L 5,000 ^<J 0,000 ^< 20,000 



ONE-THIRD OCTAVE-BAND CENTER FREQUENCIES 

FIGURE 6. - Noise produced by standard auger cutting head. 



42 



vibrational response to coal-cutting ex- 
citation forces. The first natural vi- 
bration frequency of the standard helix 
was found to be about 200 Hz. Below this 
frequency, mass and stiffness of the 
helix determine its vibrational response; 
above 200 Hz , the amount of damping at 
its higher "resonant" vibration frequen- 
cies controls its ability to vibrate. 

Radiation efficiency is a measure of 
a structure's ability to convert vibra- 
tion into airborne noise. Laboratory 
analysis of the vibration of the auger 
helix showed that radiation efficiency 



increased with increasing frequency; be- 
low 300 Hz the energy transfer was very 
inefficient. Although the coal-cutting 
forces exciting the auger were predom- 
inantly low frequency (fig. 4), signifi- 
cant noise was produced at higher fre- 
quencies because of the resonant vibra- 
tion modes of the helix and the greater 
radition efficiency. A plot of noise 
versus frequency obtained during labora- 
tory and in-mine cutting tests (fig. 6) 
showed that the standard auger cutting 
head produced both low-and high-frequency 
noise. 



DESIGN OF REDUCED-NOISE AUGERS 



The design of the reduced-noise augers 
was governed by the need to reduce their 
vibrational response to coal cutting 
forces. This was accomplished in two 
ways: (1) increasing the first natural 
frequency of the helix; and (2) increas- 
ing the amount of damping at the helix 
resonant frequencies. Because the coal 
cutting forces were predominantly low 
frequency (see figure 4) , they would only 
weakly excite a helix with a high first 
natural frequency. In addition, such a 
helix would also have fewer resonant .fre- 
quencies in the excitation range. The 
simplest way to increase the first 
natural frequency of the helix was to 
make it stiff er and heavier. Damping 
materials were then added to attenuate 
its vibration at its higher resonant 
frequencies. 

Before a stiff ening-and-damping treat- 
ment could be applied to the standard 
auger cutting head, the bit less ("con- 
veying") helix shown in figure 5 had to 
be removed. Although laboratory tests 
showed that this helix did not vibrate as 
much as the bit-carrying ("cutting") 
helix, its removal reduced the auger's 
vibrating surface area by about one-half. 
Removal of the conveying helix also pro- 
vided room on the auger and reduced its 
weight, facilitating the addition of the 
stiffening and damping treatment de- 
scribed below. Subsequent underground 



tests showed that the "conveying" helix 
was not needed for effective loading and 
cleanup . 

In order to reduce the vibration of the 
remaining helix, the original 6-1/2-in 
core diameter of the standard auger was 
enlarged. Then, a unique stiffening and 
damping treatment was applied to form a 
simple, aesthetically pleasing package. 
As shown in figure 7, a hollow helix was 
created by welding a series of "stiffener 
segments" (3/8-in-thick plates) to the 



180° segment 
( V2 revolution) 



30° segment 
/L revolution) 




Helix 
(bit holders not shown) — 

^Stiffener M£$ 



Closure 
plate 



Spray nozzle 
FIGURE 7. - Sand-filled auger fabrication technique. 



^MH 



mi^mm 



^MM 



43 



core and helix, thereby forming a cavity 
of triangular cross section. The trian- 
gular cavity was then filled with sand 
and sealed. The stiff eners increased the 
first natural frequency of the helix, 
while the sand added mass and damping. 
Three separate sand cavities were needed 
to completely cover the auger cutting 
head; the largest of these was on the 
nonconveying side of the helix (fig. &4). 
Because of space constraints, the two 
smaller sand cavities were on the convey- 
ing side of the helix (fig. SB) , but in- 
mine tests showed that they did not af- 
fect the coal-carrying capability of the 
auger. 

As shown in table 1 , two different ver- 
sions of the reduced-noise auger were 
fabricated. The purpose of the "conserv- 
ative" version (fig. 8) was to achieve a 
significant noise reduction while staying 
well within the normal conveying capacity 
of the standard auger. The purpose of 
the "aggressive" version (see table 1) 
was to achieve the largest noise reduc- 
tion possible by reducing the conveying 

TABLE 1. - Conservative versus aggressive 
reduced-noise auger designs 



Core diameter in.. 

Stiffener angle (to 

helix) deg . . 

1st natural helix 

frequency Hz . . 

Frequency 1 pet . . 

Damping treatment 

Damping increase 1 . . pet . . 

1 Change from standard. 



Conserv- 
ative 



9.5 

30 

800 

400 

Sand 

400 



Aggres- 
sive 



14.0 

45 

1,200 

600 

Sand 

400 




B 


I* jJB^gSand cavity 3 


: ^v * A 


^^*Sond cavity 2 mm Jl 




^ 


^^ ^ ^ 


>*i 


^-|I25SE2831 * 


K ' 



FIGURE 8. - Sand-filled auger, conservative 
version. A, Nonconveying side of helix; B, con- 
veying side of helix. 

capacity (helix height) to its lowest 
limit. The basic difference between the 
two versions was the auger core diameter; 
table 1 lists the other differences. 
Laboratory tests predicted that the 
reduced-noise augers would be capable of 
achieving noise reductions of 8 to 10 
dBA. 



UNDERGROUND TESTS OF REDUCED-NOISE AUGERS 



The operational performance — noise re- 
duction, cutting and loading characteris- 
tics, and durability — of the two reduced- 
noise augers described in table 1 was 
evaluated by testing them in an under- 
ground coal mine for a 6-month period. 
The miners were initially very skeptical 
about the new auger designs, especially 
the cleanup capabilities of the aggres- 
sive (larger core) version. However, the 



miners were completely satisfied with the 
performance of both auger designs because 
they were almost identical to the stan- 
dard augers in terms of cutting, loading, 
and cleanup. 

More importantly, as shown in table 2, 
both auger designs resulted in signifi- 
cant noise reductions at both the jack- 
setter and the operator positions. 



44 



Because the jacksetter position was very 
close to the cutting heads, the initial 
(standard auger) noise level was higher 
(106 dBA) and the noise reduction was 
larger (8 to 10 dBA) than at the opera- 
tor's position. Conveyor and motor noise 
played an important role in the initial 
noise level at the operator's position 
(96 dBA by themselves); therefore, the 
addition of the reduced-noise augers re- 
sulted in a lesser noise reduction (6 
dBA) but virtually eliminated cutting 
noise as a contributor to overall opera- 
tor noise. At the jacksetter' s position, 
cutting noise was reduced to the point 
where its contribution was almost equiva- 
lent to the sum of the remaining noise 
sources on the miner. 



TABLE 2. - Noise reductions produced 
by sand-filled, reduced-noise augers 
in underground tests, decibels 
(A-weighted) 





Jacksetter 


Operator 


Test condition 


Noise 
level 


Reduc- 
tion 


Noise 
level 


Reduc- 
tion 


Backgroundl. . . . 
Standard auger. 
Reduced-noise 
auger: 

Conservative. 

Aggressive. . . 


93 
106 

97 
96 


NAp 
NAp 

9 

10 


96 
102 

96 
96^ 


NAp 
NAp 

6 
6 



NAp - Not applicable. 

lAll motors and conveyors operating; 
conveyor empty; cutting heads spinning in 
air. 



CONCLUSIONS AND RECOMMENDATIONS 



Although both reduced-noise auger de- 
signs resulted in substantial underground 
noise reductions, the conservative ver- 
sion was the design most readily accepted 
by mine operators. In practice, however, 
the best approach would be to install the 
helix-stiffening and sand-filling treat- 
ments without modifying the 6-1/2-in- 
diameter core of the standard auger. The 
major reasons for this recommendation are 
(1) the larger core does not increase 
helix stiffness nearly as much as the ad- 
dition of the stiff ener plates; and (2) 
fabrication of the large core is rather 
expensive and would be difficult to per- 
form in a typical mine rebuild shop. 



Instructions for converting a standard, 
two-helix auger into a sand-filled, 
single-helix, reduced-noise auger are now 
available upon request from the Bureau of 
Mines and/or MSHA. Although the overall 
diameter of the augers in the experimen- 
tal program was 28 in, the same procedure 
can be used to modify standard augers 
with diameters of 22 to 32 in. In the 
near future, the Bureau will publish a 
report containing these instructions in 
concise form, including all engineering 
drawings and data needed for modifica- 
tion. Additional information on the re- 
search program described in this paper is 
also available from the Bureau. 



45 



COAL CUTTING NOISE CONTROL 
By Mark R. Pettitt 1 and William W. Aljoe 2 



ABSTRACT 



The results of a Bureau of Mines labor- 
atory investigation of coal cutting me- 
chanics and noise is presented. Some ba- 
sic theoretical aspects of coal cutting 
mechanics and noise generation are dis- 
cussed, and the results of the laboratory 
experiments are used to formulate analyt- 
ical models of the coal cutting forces 
and noise. It is concluded that by using 
deeper depths of cut, slower cutting 



speeds, and more efficient cutting tools, 
it is possible to reduce the level of 
coal cutting noise as well as provide 
benefits to other important areas of un- 
derground health and safety. In addition 
to these basic operational parameters, 
fundamental structural design criteria 
for reduced-noise coal mining machine 
cutting heads are also presented. 



INTRODUCTION 



In response to the Federal Coal Mine 
Health and Safety Act of 1969, which es- 
tablished maximum noise 
permissible for mining 
Bureau has undertaken a 
search programs aimed 



contribution 
is smaller. 



to the overall noise level 



exposure levels 
personnel, the 
number of re- 
at reducing the 



noise associated with mining operations. 
One of the more serious noise problems in 
the coal mining industry is associated 
with the operation of continuous miners 
in underground coal production. 

A noise survey was conducted on a rep- 
resentative sample of continuous miners 
CO, 3 and a summary of these results is 
presented in figure 1. These data show 
that the vibration of the cutterhead and 
conveyor are the major continuous miner 
noise sources. The drive train and hy- 
draulic system, on the other hand, are 
secondary noise sources, because their 

Senior research engineer, Wyle Labora- 
tories, Hunts vi lie, AL. 
o 
"Mining engineer, Pittsburgh Research 

Center, Bureau of Mines, Pittsburgh, PA. 

•^Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this paper. 



The problem addressed in this paper is 
the noise generated by underground coal 
cutting operations. The noise sources 
directly associated with the cutting of 
coal are diagramed in figure 2. The 
available experimental evidence indicates 
that coal fracture noise and face radi- 
ated noise generated during the operation 
of the typical continuous miner are well 
within present MSHA noise regulations 
(2^) . The order of importance of the re- 
maining coal cutting noise sources is 
primarily determined by the design and 
operation of the cutterhead. 

Several fundamental design and opera- 
tional concepts for low-noise coal cut- 
ting are presented in this paper. These 
concepts apply to any type of continuous 
miner that uses bits to break coal from 
the face. Understanding how the design 
concepts result in low-noise coal cutting 
requires a limited background in struc- 
tural dynamics. The "Background" section 
should provide the needed basics for the 
uninitiated reader. 



46 



BACKGROUND 



The fact that a structure will vibrate 
when struck and that the vibration even- 
tually stops is a matter of common exper- 
ience. Structural dynamics is the term 
used to describe the area of scientific 
study that seeks to mathematically de- 
scribe this common experience. The way 
in which a structure vibrates in response 
to an applied force is an inherent at- 
tribute of the structure. 



< 
GQ 

-a 

*» 
_l 
UJ 

> 

UJ 

_J 

LU 
OC 
3 
CO 
CO 
LU 
OC 
Q_ 



3 
O 
CO 



105 



100 - 



95 



90 



85 - 



80 



75 



y% /Conveyor 
' only 



Cutting only 



Tull 
operation 




Allother 
sources- 



Average for 
Jeffrey 
Joy 

Lee Norse 
National Mine 
Service 



NOISE SOURCE 



FIGURE 1. - Contributions of major sources 
of noise for continuous miners. 



The response of a structure can be more 
easily examined in the frequency domain. 
Frequency is a period of oscillation or 
the rate at which specific action or 
event repeats itself. If an event or 
motion is repeated 20 times a second, the 
frequency of oscillation is said to be 20 
Hz. A pure tone of music is a pressure 
variation in the air that strikes the ear 
at regular intervals. Any signal time 
history, such as the force time history 
in figure 3, can be thought of as the sum 
of an infinite number of single frequency 
signals. Each frequency has a specific 
amplitude and starting time relative to 
the other frequencies. To continue with 
the music or sound example: If the force 
signal of figure 3 were a noise signal, 
it could be exactly re-created by a very 
large number of whistlers. Each whistler 
would be assigned a specific tone or fre- 
quency to whistle, a specific loudness to 
whistle, and a specific time to start 
whistling. If done just right, the sum 
of all the whistlers' single frequency 
inputs would result in the specific noise 
time history desired. 



EXCITATION 



NOISE SOURCE 
MECHANISM 



CHARACTER 
OF NOISE 



Individual 

bits engage 

coal face 



X 



Cutting bit 
reaction loads 

initial 

impact force 

fluctuating 

cutting forces 



Cutting bit- coal 

interaction 

produces coal 

fracturing 



^^ 



Coal 

fracture 

noise 



Cutting bit-coal 

interaction 
produces coal 
face vibration 



Face 

radiated 

noise 



Force 

transmitted to 

cutterhead 



Vibrational 
response of 
cutterhead 



Cutterhead 

radiated 

noise 



Vibrations 

transmitted 

to miner 



Vibrational 
response 
of miner 

components 



Miner 

radiated 

noise 



FIGURE 2. - Coal cutting noise sources. 



47 



,000 



-o 500 



UJ 

o 

a: 
o 

U- 







500 



Peak force = 633 lb 



Mean cutting force = 309 I b 




^ '■* W V 



25 

FIGURE 3. 



50 



150 



75 100 125 

TIME,ms 

Force time history of single auger bit cutting coal at 60 in/s. 



175 200 



N 

X 
CD 



90 



80 1 



70 



Q O 

o w 
UJ 

85 

uj 

o 

°- 40 



30 



20 



1 M 



40 



1 ' I ' I ' I 




i I i 



100 200 400 1,000 2,000 4,000 10,000 

FREQUENCY, Hz 

FIGURE 4. - Force power spectral density of single auger bit cutting coal at 60 in/s. 



48 



Of course, the signal does not have to 
be noise. The signal can be force, ve- 
locity, displacement, or any measurable 
quantity. The frequency domain represen- 
tation of a time history simply describes 
the magnitude that each frequency compo- 
nent must have and the time at which the 
frequency must begin relative to the 
other frequencies. The force time his- 
tory of figure 3, therefore, is made up 
of the frequency domain force components 
of the magnitudes shown in figure 4. 



over a range of frequencies , the frequen- 
cy response characteristics of the struc- 
ture are, of course, more fully defined. 
Figure 5 shows the response characteris- 
tics at the free end of a beam fixed at 
one end (like a flagpole). The charac- 
teristics are determined by the specific 
configuration of the structure. Perhaps 
the greatest emphasis in structural dy- 
namics is the development of methods to 
predict the response characteristics of a 
structure. 



The vibratory response of a structure 
to an applied force is strongly dependent 
on the frequency at which the force is 
applied. The response characteristic of 
a structure at a specific frequency is 
found by measuring the motion caused by a 
unit force applied to the structure at 
that frequency. If measurements are made 



It is clear in figure 5 that the 
structural response to a unit force is 
much greater at some frequencies than 
at others. This phenomenon is an im- 
portant aspect of structural dynamics. 
It results from the combined effects 
of the distributed mass and stiffness 
of a structure. At these high-response 



o> 



O 

ZD 
U_ 

q: 
uj 
u_ 
en 

-z. 
< 



< 

UJ 

_l 
UJ 

o 
o 
< 




250 
FREQUENCY, Hz 



500 



FIGURE 5. - Structural response characteristics. 



49 



frequencies, the motion of the structure 
is limited only by its energy dissipation 
(damping) characteristics. The frequen- 
cies at which this occurs are called the 
"natural" frequencies, or resonances, of 
a structure. The response at the natural 
frequencies of a structure can easily be 
100 times the response of the structure 
to the same force magnitude applied at a 
nonresonant frequency. When one pushes a 
child in a swing, one applies a periodic 
force at the first natural frequency of 
the swing. The energy dissipation mech- 
anism of the swing is primarily the wind 
drag against the body as it moves through 
the air. One final example is a flag- 
pole. One can cause the tip of the flag- 
pole to swing widely from side to side 
by pushing on the pole at a specific 
rate — the rate at which the pole natural- 
ly sways back and forth. The energy 
dissipation (damping) of the pole lim- 
its the peak deflection of the pole and 
causes the pole to come to rest after 
the periodic force is removed. All 



structures have an infinite number of 
natural frequencies. The frequencies 
at which these resonances occur is sole- 
ly a function of the structural con- 
figuration (mass, stiffness, and damping 
distribution) . 

That a vibrating structure cruses noise 
is another fact of common experience. 
Some of the energy of vibration is ex- 
pended moving air. The amount of energy 
transferred to the air from a vibrating 
structure is proportional to the average 
surface velocity of the structure. The 
proportionality is the efficiency at 
which the vibrating structure can pass 
energy to the surrounding air. It is 
often called the radiation efficiency. 
The radiation efficiency is a strong 
function of both frequency and structural 
configuration. It tends to be quite 
small at lower frequencies and rises to a 
constant maximum value as the frequency 
increases. 



NOISE CONTROL CONCEPTS 



Each noise control concept presented in 
this paper takes full advantage of the 
basic nature of the dynamic forces gener- 
ated during coal cutting to reduce coal 
cutting noise. Continuous mining ma- 
chines exploit the brittle fracture char- 
acteristics of the coal. When a cutting 
bit engages the coal face, the coal re- 
sists its advance in a manner analogous 
to a spring. The applied force increases 
until the stress in the coal initiates 
localized brittle fracture. The coal 
fracture propagates for some characteris- 
tic distance from the point of force ap- 
plication. The cutting bit continues to 
advance, meeting steadily decreasing re- 
sistance as it clears out the fractured 
coal. When the bit again contacts un- 
fractured coal, the process is repeated. 

A representative force time history 
(fig. 3) and force power spectral density 
(PSD) (fig. 4) of a rigidly mounted bit 
cutting coal clearly show the highly im- 
pulsive nature of the process. It is in- 
teresting to note that the impact force 



generated as the bit initially enters the 
coal is no more impulsive than the subse- 
quent individual fracture events. The 
extensive experimental program from which 
figures 3 and 4 were taken has shown that 
both the stress required to initiate 
fracture and the characteristic distance 
of fracture propagation are relatively 
independent of cutting bit velocity. 
Hence, for a given type of coal, depth of 
cut, and bit configuration, the peak 
force that initiates coal fracture and 
the total number of fracture events that 
occur in a prescribed length of cut are 
velocity independent. 

All continuous mining machines experi- 
ence this type of excitation. In the 
frequency domain, the cutting force PSD 
is flat until a cutoff frequency, after 
which it declines with frequency at a 
rate that is inversely proportional to 
the square of the frequency (see figure 
4) . The cutoff frequency is primarily a 
function of the coal type and the cutting 
bit velocity. 



50 



The peak and mean cutting forces are a 
function of the material being cut, bit 
type, and depth of cut. While not exten- 
sive, cutting force data have been ob- 
tained for a variety of coal, shale, and 
rock types at several different depths of 
cut 0~6_) • Table 1 summarizes the gener- 
ally available data from both laboratory 
and in situ cutting tests. 

Recall the essential characteristics of 
coal cutting: First, coal resists ad- 
vance of the bit in a manner analogous to 
a spring. Second, localized brittle 
fracture occurs when the stress in the 
coal reaches a critical value. Third, 
the brittle fracture propagates for some 
characteristic distance from the point of 
force application. Fourth, bit velocity 
does not affect the first three charac- 
teristics. Of course, since real coal is 
not homogeneous , the above is only true 
in a very rough statistical sense. These 
characteristics are represented in the 



very simple but quite useful single- 
degree-of-freedom coal model illustrated 
in figure 6. 

Assuming a constant bit velocity, Vt>, 
the model results in a sawtooth cutting 
force, F c , time history. A more accurate 
model is achieved if, instead of allowing 
the spring to go discontinuously to zero 
when the cutting force equals the frac- 
ture force, Ff , a steadily decreasing 
cleanup force is provided until the next 
coal spring is encountered by the bit 
(F c =Ff-K'AX' , where K' is the spring con- 
stant after fracture and AX' is the dis- 
tance of the bit tip from the point of 
fracture) . The modified model produces 
the more triangular pulse shape seen in 
the actual cutting force time history. 
The model with experimentally determined 
constants, K c , K' , l c and Ff , can be used 
to evaluate certain cutterhead operation- 
al parameters and design concepts. 



TABLE 1. - Laboratory and in situ cutting force data 



Material 



Bit type 



Depth 

of cut , 

in 



Mean 

force, 

lbf 



Standard 
deviation, 
lbf 



Peak 
force, 
lbf 



Blue Creek coal, 



Pittsburgh No. 1 seam coal. 



Pittsburgh No. 2 seam coal, 



Illinois No. 6 seam coal, 



Illinois shale , 

Bruceton synthetic shale, 



Blue Creek shale, 



Sandstone, 



Plumb bob. 
• . .do.'. . . , 
. . .do. . . • , 



• • • QO •••••••• 

. . .do 



• • • QO ••••••••••••• 

. . .do 

. . .do 



...do, 
. . .do, 
...do, 



Wedge, 



...do, 
. . .do, 



Plumb bob 

Auger point attack 

Wedge type 1 

Wedge type 2 

Point attack 



0.5 

1 

2 

.5 



.5 



.5 



.5 



.5 



607 

925 

1,500 

515 
958 

773 
1,037 
1,489 

694 
1,112 
1,310 

866 

616 
1,739 

925 
650 
689 
667 
1,385 



325 
541 
950 

245 
590 

406 
646 
929 

329 
525 
642 

673 



1,608 
1,994 
3,309 

1,082 
2,440 

2,088 
2,727 
3,679 

1,704 
2,495 
2,555 

2,918 



472 


3,163 


127 


6,780 


541 


1,994 


337 


1,494 


533 


3,330 


511 


2,303 


NA 


NA 



NA Not available, 



51 



Force, Fc 



Typical fracture event 





Fracture 
force, Ff 



Distance, X 



Fc Equivalent to spring with : 




$ K C = K C , Fc^Ff 



K c =0; F c >Ff 



Fracture length 



FIGURE 6. - Coal cutting force model. 
NOISE CONTROL BY REDUCING BIT VELOCITY 



The first principle of cutterhead de- 
sign and operation is: Keep the bit 
velocity as low as possible. The charac- 
teristic fracture length as described in 
the coal cutting model is independent of 
velocity. Therefore, the impulse time 
(or duration) of an individual fracture 
event is inversely proportional to the 
bit velocity. The typical impulse time 
determines the cutoff frequency of the 
force spectrum. The shorter the impulse 
time, the higher the cutoff frequency. 
Experiments with a wide range of coal, 
shale, and synthetic coals have shown 
that the one-third octave-band force lev- 
el decreases about 1.5 to 2 dB per band 
after the cutoff frequency (fig. 7). 
This is typical of the spectrum associ- 
ated with a triangular pulse shape. 
Slower cutting, therefore, lowers the 
high-frequency force levels by reducing 
the cutoff frequency. 

Several factors combine to make the low 
frequency (below 200 Hz) dynamic cutting 
forces of little importance as a source 
of harmful radiated noise. Below 200 Hz, 



most cutterhead and miner structures are 
generally in the stiffness-controlled 
region of their structural response. 
Hence, the magnitude of the structural 
vibration due to a given unit of force is 
not significantly amplified. The trans- 
fer of energy from the vibrating struc- 
ture to the air is also very inefficient 
in the frequency range below 200 Hz for 
the typical cutterhead, miner size, and 
configuration. Finally, the noise that 
is radiated below 200 Hz is not as damag- 
ing to the ear as the higher frequency 
noise. The A-weighting of noise spectra 
(fig. 8) is typically used to represent 
this fact. MSHA workplace noise regula- 
tions are written in terms of A-weighted 
noise. 

The bit velocity can be reduced while 
maintaining coal production by increasing 
the depth of cut and/or the number of 
bits per cutting line. The hypothetical 
characteristics given in table 2 and the 
following paragraph illustrate and ex- 
plain the noise control advantage of re- 
duced bit velocity. 



52 




100 200 400 800 1,600 3,150 6,300 12,600 Overall 

125 1^250 1^500 U,000__L 2,000 1 4 i 000 k.^ 000 L 16,000 



131.5 l 63 I 125 L250 L500 11,000 L 2,000 L 4,000 18,000 L 16,000 

\M0 ^^80 \il60 ^s3!5 ^^630 ^^250^^500^4^000 x; ^(X)0 v; <20,000 



FREQUENCY, Hz 

FIGURE 7. - Typical one-third octave band coal cutting force spectrum. 



TABLE 2. - Hypothetical drum 
characteristics 



Cutting diameter in. . 

Speed rpm. . 

Bits per cutting line 

Depth of cut (max.) in.. 



40 

60 

1 

1 



B 



40 

30 

] 2 

1 



40 

30 

1 

2 2 



^-start drum. 
2 Doubles force level on the bit over 
the force level of a 1-in-deep cut. 

Assume a baseline or control cutter- 
head, A, with the characteristics listed 
in table 2. A comparable cutterhead, B, 
would have the same depth of cut as drum 
A but operate at one-half the bit veloc- 
ity. A cutterhead that must operate at 
twice the depth of cut of drum A to main- 
tain the same production would have to 
have the characteristics listed for drum 
C in table 2. The force spectrum act- 
ing on the bits of each drum type is 



drum type is postulated in figure 9. The 
force scale of the figure is arbitrary 
since it is the relative force magnitude 
of the three drums that is of interest. 
The dynamic force on each bit of drum B 
is 6 dB less than on each bit of drum A 
in the frequency region above the cutoff 
frequency of drum A. Reduced force on a 
cutting bit translates directly to re- 
duced miner structural vibration due to 
that bit. The total vibration of a con- 
tinuous mining machine is the sum of the 
vibration caused by each bit that is in 
contact with the coal. Since drum B has 
twice the number of bits as drum A, a 6- 
dBA reduction in the force on each bit 
would result in a 3-dBA overall struc- 
tural vibration and noise reduction. 

Drum C has the same number of bits as 
drum A. Drum C operates at one-half the 
cutting speed but twice the cutting depth 
to maintain the same production rate as 



53 



10 r 



DO 
"O 



O 

Ld 

(T 
O 
O 




-20 - 



-30 - 



32 63 



125 250 500 1,000 2,000 4,000 8,000 
FREQUENCY, Hz 

FIGURE 8. - A-weighting of noise spectra. 



drum A. As noted in table 2, the cutting 
force magnitude of drum C is assumed to 
double with a doubling of cutting depth. 
This is actually somewhat conservative 
because data (table 1) often indicates 
only a 1.5-to 1.7-fold increase in force 
per doubling of cutting depth. The 3-dB 
force reduction (fig. 9) achieved above 
the cutoff frequency of drum A should re- 
sult in a 3-dBA reduction in coal cutting 
related noise. 

Reference 3 contains quantitative ex- 
perimental support for this analysis 
method. Coal cutting noise, as deter- 
mined in the laboratory, for a 1-in depth 
of cut and 96-in/s cutting speed averaged 
101.5 dB as measured on a flat (non- 
A-weighted) scale. Coal cutting noise 
for a 1-in depth of cut and 16-in/s 



cutting speed averaged 88.2 dB, a 13. 3-dB 
reduction. Based on the cutting speed 
reduction, the model presented here pre- 
dicts a force reduction of 12 to 16 dB 
above the cutoff frequency of the 96-in/s 
cut. Again, because the A-weighting de- 
emphasizes the lower frequency force com- 
ponents , the force reduction should re- 
sult in a 12-to 16-dBA noise reduction. 

Although an overall noise reduction of 
around 3 dBA is not sufficient to solve 
all cutterhead noise problems , the reduc- 
tion can be achieved for very little 
cost. In addition, there is strong evi- 
dence that slower and/or deeper cutting 
produces less dust and also reduces the 
likelihood of spark ignitions of methane 
(7-9). 



54 



40 



30 



GO 
T3 

LU 
O 

o 

Ll. 



20 - 











-10 




l,OUU 0,I3U o,ouu i<:,ouu 

L 2,000 V^iOOO C 8,000 L 16,000 

^so^^^otr^^oo^^^tr-^OOjOoo 



FREQUENCY, Hz 



FIGURE 9. - Estimated force spectra. 
NOISE CONTROL THROUGH CUTTERHEAD STRUCTURAL RESPONSE ALTERATION 



The dynamic forces at each bit-coal in- 
terface are reacted by the cutterhead 
structure. The resulting vibration of 
the cutterhead structure is generally the 
major coal cutting noise source. The 
second principle of cutterhead design is: 
Keep the frequency of the first struc- 
tural mode as high as possible. The 
third principle of cutterhead design is 
also related to the response characteris- 
tics of the cutterhead structure: Keep 
the structural damping as high as possi- 
ble. Because the excitation also passes 
through the cutterhead to the other miner 
components, these rules apply to all 
miner structures. The purpose of the 
rules is tq reduce the resonant response 
of the structures to the dynamic excita- 
tion of the coal cutting forces. 



Because the frequency spectrum of the 
cutting force decreases rapidly as the 
frequency increases , the coal cutting 
noise above about 2,000 Hz is generally 
below the level of concern. Indeed, 
noise below the 1,000-Hz band generally 
controls the overall level. The second 
design principle limits the structural 
response to the cutting forces primarily 
by eliminating structural resonances in 
the "problem" frequency range. Those 
structural resonances that cannot be 
eliminated from the band of concern 
should be well damped (design principle 
3). Damping, of course, limits the mag- 
nitude of the structural response at the 
resonances. 



55 



NOISE CONTROL THROUGH ISOLATION OF RADIATING STRUCTURES 
FROM THE DYNAMIC CUTTING FORCES 



The dynamic cutting forces cause vibra- 
tion not only of the cutterhead but also 
of the other miner components. The vi- 
brations propagate throughout the mining 
machine. The fourth principle of cutter- 
head design is: Keep the higher frequen- 
cy dynamic cutting forces isolated from 
as much of the miner structure as possi- 
ble. As previously discussed, the higher 
frequency coal cutting forces cause the 
unacceptably high noise levels. The 
higher frequency coal cutting forces can, 
in principle, be isolated from the major 
radiating surfaces of the cutterhead and 
the miner. This has been demonstrated by 
both analysis and experiment. Two ana- 
lytical models have been developed under 
an ongoing Bureau contract (10) . The 
first model was capable only of evaluat- 
ing the feasibility of the concept. The 
second model allowed the detailed eval- 
uation of candidate isolated cutting 
tool designs. The analytical feasibility 
evaluation involves a rough approximation 
of the cutting force characteristics in 
combination with simple frequency domain 
force isolation mathematics. 

The peak forces seen in the force-time 
history of figure 3 produce the stress 
required to initiate brittle fracture of 
coal. The peak force, not the mean 
force, cuts the coal. Therefore, if coal 
cutting is to occur, the force-time his- 
tory at the tip of an isolated cutting 
tool should be very similar to that of a 
rigidly mounted tool. The force peaks 
must be reached. The initial loading 
rate of the bit tip at each fracture 
event may be somewhat smoothed by the 
softer response of the isolator. This 
effect is, however, limited by the number 
of fracture events that must occur in a 
given length of cut. Therefore, the 
force on the bit tip can be considered, 
in this rough analysis, to be independent 
of the response characteristics of the 
cutting tool. 



A simple single-degree-of-freedom iso- 
lated cutting tool is shown in figure 10. 
Assume that — 

The mining machine has sufficient 
power to maintain a constant cutting 
head velocity, V^ , regardless of the 
force generated at the bit-coal inter- 
face, F c . 

The cutting head structure is rigid 
compared to the isolator. 

The cutting bit and tool holder are 
rigid compared to the isolator. 

The isolator is massless. 

A frequency domain dynamic force bal- 
ance can be written to calculate the 
force transmissibility, T, of the iso- 
lator. Force transmissibility at any 
given frequency, co(w=w 2 ir f ) , is defined as 
the ratio of force at the base of the 
isolated cutting tool to the force at the 
tip. With the definition of symbols as 

in figure 10, the result is 

1/2 



T = 



(K 2 + oo 2 C 2 ) 2 



(K 2 + w 2 (C 2 - mKj)) 2 + (w 3 mC) 2 



This equation can be used to evaluate the 
gross feasibility of the isolated cutting 
tool. It also clearly demonstrates the 
concept and advantages of force isola- 
tion. Figure 11 gives the transmissibil- 
ity at various levels of damping over a 
range of frequency ratios, f/f n » where 
f is the exitation frequency and f n is 
the natural frequency of the isolated 
cutting tool. The isolated cutting tool 
simply allows the mass and acceleration 
of the tool to react a portion of the 
coal cutting force. 



56 



Tool driver 



vxmmm&>$m\ 




Isolated cutting tool 
Ki 



C 



M 




! v h 



Vc 



M 
V C 



KEY 

Ki Isolator spring stiffner 
C Isolator damping coefficient 
Vh Velocity of cutting head 
Fh Force at cutting head 

Mass of isolated cutting tool 

Velocity of cutting tool tip 

Force at the cutting tool - coal interface 

FIGURE 10. - Single-degree=of-freedom iso- 
lated cutting tool. 

A mineworthy isolated cutting tool must 
have the following characteristics: 

1. The highest natural frequency of 
the tool isolation system must be less 
than 170 Hz to assure that isolation oc- 
curs above 250 Hz. 

2. Elastomeric isolator elements must 
not be allowed to deflect more than 10 
pet of the isolator thickness under shale 
cutting force levels. 

3. Cutting tool space constraints must 
not be violated. 

The first characteristic assures that 
the frequency region of force isolation 
begins before the problem frequency band. 
The second characteristic is required to 
extend the life of the elastomeric iso- 
lator elements. The third characteristic 
is an obvious requirement. It is most 
difficult to meet when the prototype 
reduced-noise cutter-head must fit on 
present machinery. 



>- 



m 

CO 
CO 



CO 

< 

h- 




I /2~ 2 3 4 ! 
FREQUENCY RATIO, f/f n 
FIGURE 11. - Typical transmissibility curves. 



For the purpose of rough feasibility 
calculations, assume a maximum allowed 
isolated cutting tool deflection, d m of 
0.15 in, damping, £, of 8 pet and natural 
frequency, f n , of 170 Hz. Table 3 gives 
details on shale cutting force (5_) . The 
design values are the maximum force 
levels measured during the experimen- 
tation. Because the excitation of the 
isolated cutting tool is a continuous 
dynamic process, the peak deflection 
of the isolator will occur at the reso- 
nance frequency, co n . The transmissibil- 
ity equation and the equations defining 
co n (w n = /Kj/m) and £<£ = C/2 /K|m) can be 
used to help calculate the peak deflec- 
tion of the isolator at resonance. At 
resonance, the transmissibility equation 
reduces to 



_ .. 2/Kim 1 
1 " C " 2C ' 



Using the design parameters given in ta- 
ble 3, the total equivalent peak static 
force, Fg, on the isolator at resonance 
is 

F E = (FB * CF * 1/2 ) + F m 

= 15,000 lbf. 



TABLE 3. - Shale cutting parameters for feasibility calculations 



57 



Parameter 



Peak force , F p lbf . . 

Mean force , F m lbf . . 

Standard deviation of resultant cutting force, a lbf.. 

Peak force to root-mean-square force ratio, CF 

Root-mean-square force in band of width (2z;f n ) centered at 

fn, F B lbf.. 

Equivalent static force, F E lbf.. 



5 Assumed damping, 8 pet. 
1 Average. 




Design 



f n Assumed resonant frequency, 170 Hz. 



7,800 

1,500 

1,400 

3.6 

525 

15,000 



The isolated cutting tool mass, m, needed 
to meet the dynamic conditions assumed 
above can be calculated by 



m = 



m = 



m = 



(F E /d m ) , 
(2Tff n ) 2 

(15, 000/0. 15) ? 
(2tt170) 2 

0.09 lbf s 2 /in (33 lb) 



These simple calculations indicate that 
the isolated cutting tool concept is 
quite feasible and that the concept de- 
serves much more rigorous scrutiny. 

A more complex model has been developed 
to aid in the detailed design and evalua- 
tion of isolated cutting tools. The ana- 
lytical model includes rigid body cutting 
tool motion in all translational and ro- 
tational directions. Any number of iso- 
lators are allowed by the computer pro- 
gram that implements the model. The user 
simply tells the program where the elas- 
tic center of each isolator is located 
and supplies the stiffness and damping of 
the isolator in the coordinate directions 
best suited to the isolator. The program 
calculates the required global stiffness 
and damping vectors from the local iso- 
lator specific values. The total mass 
and the rotational inertia of the cutting 
tool can be approximated by the combina- 
tion of several simple geometric shapes 
to match the more complicated shape of 
the cutting tool. The program combines 
the known mass and rotational inertias of 



the simple shapes to obtain the total 
cutting tool mass and global coordinate 
system inertias. With these inputs, the 
program calculates the rigid body natural 
frequencies, the frequency domain cutting 
force transmissibility, and the displace- 
ment spectrum of the cutting tool. 

In addition to the frequency domain an- 
alysis, the program has time history 
analysis capability. A coal seam model 
that may include layers of rock can be 
defined. The basic coal and rock model 
are as previously described. The time 
history model maps the deflection of all 
points of interest through a full rota- 
tion of a cutterhead of specified cutting 
diameter, advance rate, and rotational 
speed. The program is a fast and effec- 
tive way of evaluating and optimizing the 
response characteristics of an isolated 
cutting tool design. 

The first experimental evidence that 
isolated cutting tools could cut coal and 
achieve significant isolation was ob- 
tained with a linear cutting apparatus 
(LCA). The LCA (fig. 12) was designed 
specifically for controlled coal cutting 
force measurements. An isolated cutting 
tool was built for mounting on the LCA 
(fig. 13). A rigid cutting tool was also 
built (fig. 14). Coal was cut at 60 in/s 
from opposite sides of the same coal sam- 
ple (fig. 15). One side was cut with the 
rigid tool and the other side with the 
isolated tool. Cuts were made on several 
blocks of coal in this manner. 



58 




60 _ durometer 
elastomer — 




30 _ d urometer 
elastomer- 



tzzzzzzm 



m?' 



wm 



lb mass 



ELASTOMER SPRING CONSTANTS 



9,075 lb/ in 

4,500 lb/in 



2,933 lb /in 

675 Ib/i 



n 



— 480 lb/in 



z 
5 lb/in 



FIGURE 12. - Linear cutting apparatus (LCA). FIGURE 13. - Isolated cutting tool for LCA tests. 




FIGURE 14. - Rigid cutting tool for LCA tests. FIGURE 15. - Typical coal sample for LCA tests. 



59 



2,500 



KEY 
Rigid cutting bit 



Isolated cutting bit 








20 40 60 80 100 

TIME, s x I0 3 

FIGURE 16. - Force time histories for LCA tests. 



20 140 



CD 



c/) 

■z. 
< 
or 

l- 




-40 



500 750 

FREQUENCY, Hz 

FIGURE 17. - Isolator transmissibility for LCA tests. 



1.000 



60 



Figure 16 gives the force time his- 
tories experienced at the cutting tool 



mount for both tools. The force trans- 
mitted to the mount by the isolated cut- 
ting tool is clearly smoother. Figure 17 
gives the frequency domain results in the 
form of isolator effectiveness (force at 
base for isolated tool divided by the 
force at the base for the rigid tool) . 



The curve is rough because of the statis- 
tical differences between coal along each 
separate cutting line. The weight and 
particle size distribution of the coal 
cut by the isolated bit were not statis- 
tically different from those of the rig- 
idly mounted bit. These analytical and 
experimental results clearly demonstrate 
that coal cutting forces can be isolated. 



APPLICATION 



Recall the three basic coal cutting 
noise control concepts discussed in this 
paper: (1) reducing bit velocity, (2) 
altering the structural response of the 
cutting head, and (3) isolating the cut- 
ting head from dynamic cutting forces. 
The first concept, reducing bit velocity, 
can be applied to almost any continuous 
mining machine provided that other design 
changes (e.g., deeper cutting, bit mount- 
ing design, etc.) are made to maintain 
production levels. The second and third 
concepts have been applied in several 



recent and ongoing Bureau research pro- 
grams , as described in the following 
section. 

ALTERING THE STRUCTURAL RESPONSE 
OF THE CUTTING HEAD 

Auger Miner Cutting Head (11) 

The helix of the standard auger miner 
cutting head (fig. 18) is the primary 
coal cutting noise source of the auger 
mining system. Following the structural 




FIGURE 18. - Standard auger miner cutting head. 



61 



design precepts, the helix was stiffened 
and damped. The dual goals of achieving 
a high first natural frequency and high 
damping of the auger helix were obtained 
with a sand-filled, conical-helix stiff - 
ener. The stiffener can be seen on the 
prototype reduced-noise cutting head in 
figure 19. The cavity created by the 
stiffener and the helix was filled with 
sand and sealed. To further stiffen the 
helix, the core size was increased. This 
configuration achieved a fourfold in- 
crease in helix first natural frequency 
and a tenfold increase in helix damping 
at the resonances below 2,000 Hz. The 
full operational capability of the design 
was verified by a 6-month-long in-mine 
test at Mears Coal Co., Dixonville, PA. 
In-mine noise measurements, taken during 
the operational testing, demonstrated a 
10-dBA noise reduction at the worker 
position nearest the cutterhead. Con- 
trolled laboratory cutting tests on syn- 
thetic coal verified the in-mine noise 
reduction results (fig. 20). 

Longwall Shearer Cutting Head (12) 

The longwall shearer cutting head is 
very similar to that of the auger miner 
in that the spiral, platelike helix is 
the primary coal cutting noise source of 
the mining system. Therefore, the long- 
wall shearer helix was stiffened and 
damped in the same manner as the auger 
cutterhead helix. Although, the first 
resonance of the helix was significantly 
increased, the damping technique only 
slightly increased the damping of the 
very massive longwall helix. Laboratory 
tests predicted a noise reduction of 4 
dBA for the stiffening and damping proce- 
dure. Although significant, this noise 
reduction is not sufficient to eliminate 
coal cutting noise as a potential long- 
wall workplace hazard. 

The damping of the longwall shearer 
helix could possibly be doubled by the 
use of a different damping material, thus 
resulting in an additional 3-dBA re- 
duction in the noise radiated by the 
helix. Because of the helix configura- 
tion, the first resonance of the helix is 



probably near the practical maximum. If 
significantly larger noise reductions are 
to be achieved, therefore, it is evident 
that a radically different shearer head 
design is required. 

The helix of the traditional longwall 
shearer head performs two functions : It 
supports the cutting bits in the standard 
helical pattern, and it moves the cut 
coal to the conveyor. The dual function 
forces a configuration that is inherently 
highly resonant and lowly damped in the 
"problem" frequency range (200 to 2,000 
Hz). As previously discussed, attempts 
to stiffen and damp the helix results in 
only moderate noise reductions. The ba- 
sic geometry limits the increase in 
stiffness and resonant frequency that can 
reasonably be achieved. Increases in 
damping are limited by both geometry and 
material properties. 

A new concept in bit lacing has been 
developed to eliminate the necessity of a 
continuous, two-function steel helix. In 
this lacing concept , called staged bit 
lacing, the bits are arranged in sets of 
cutting bit arrays. Each cutting bit 
array consists of several bits mounted on 
a single stand. The bits of each cutting 
array are set adjacent to each other. 
The individual stands have no natural 
frequencies within the range of interest. 
Each stand is very bulky (that is, all 
dimensional ratios approach unity) , which 
further reduces the efficiency at which 
the vibrational energy of the structure 
is passed to the air. 

The conveying function of the steel 
helix is performed by a helix of wear- 
resistant, high-density polyurethane . 
The material selected is sufficiently 
stiff to bridge the distance between the 
bit array stands but has extremely high 
damping. It will not support significant 
resonant vibration. The material has 
twice the wear resistance of wear- 
resistant steel; in fact, it is used as 
conveyor lining and at chute transfer 
points in many industries, including coal 
preparation plants. 



62 





FIGURE 19. - Reduce-noise auger miner cutting head. 



63 





110 


< 




GO 
T3 


100 






LU 




> 


90 


LU 




_l 




UJ 




q: 


80 


3 




if) 




ft 




QT 


70 


Q. 




Q 




•z. 




3 


60 


O 




if) 





50 



i — i — i — i i i " i i i i i i i i i i i i — m — i — i — i — i — i — i — i — i — i — r~r 



Standard auger 105 dBA 



New design auger 
94.5 dBA 





':'^/^}'i'~^' :, ?:^'f , yj'.'^^'^ 



•■■".■"■"J t.j 

m 



J I I I I I I I I I I I I I I I I I I I I I I l_L 



25 50 100 200 400 800 1,600 3,150 6,300 12,600 

31.5 ^63 Gas ^250 ksoo k 1,000 k 2,000 k 4,000 k8,ooo kj6,ooo Overall 

40 \80 \I60 \3I5 \630 \ 1,250 \2,500\ 5,000\ I0,Q00\ 20,000 




FREQUENCY, Hz 

FIGURE 20. - Noise from laboratory test of auger miner cutting head. 



This structurally altered shearer head 
(staged bit lacing and nonmetallic helix) 
has been fabricated and is now being 
tested at the Bureau's Pittsburgh (PA) 
Research Center. Extensive in-mine tests 
of the new shearer head design are also 
planned. 

Isolation of Coal-Cutting Forces (10) 

Figure 21 shows the basic design con- 
figuration of an isolated cutting head 
for a drum-type continuous miner. The 
"cutting-drum" actually consists of 12 
separate 7-in-wide, 22-in-diameter rings. 
The rings are mounted side-by-side around 
a 7.5-in-diameter central drive shaft to 
form a cylindrical, drum-shaped cutting 



head. Three to five bit blocks can be 
mounted on the outer surface of each ring 
in either a standard scroll lacing pat- 
tern or, as shown in figure 21, in a 
side-by-side (staged) lacing pattern. 
The rings are mounted to the central 
drive shaft through elastomeric bushings 
and are separated from each other by 1/2- 
in air gaps. The bushings isolate the 
cutting head (and the remainder of the 
machine) from all coal cutting exitation 
forces above 140 Hz. Theoretically, this 
would render coal cutting noise insignif- 
icant when compared with noise from other 
sources on the miner (see figure 1) . 
This cutting head is now being fabricated 
for testing in an operating underground 
mine. 



64 





SECTION A 



SECTION B 





PLAN VIEW 
FIGURE 21. - Continuous miner cutting head with isolated cutting tools. 

CONCLUSION 



SIDE VIEW 



Investigations into the mechanics and 
noise associated with coal cutting have 
revealed three basic methods for reducing 
coal cutting noise. First, the cutting 
bit velocity should be kept as low as 
possible; second, the cutting head struc- 
ture should have a very high natural fre- 
quency and be highly damped; third, the 
coal cutting forces should be isolated 



from the remainder of the cutting head 
structure. The first method can be used 
on any type of coal cutting machine. The 
second method has been used successfully 
by the Bureau on an auger-type continuous 
miner and is now being tried on a long- 
wall shearer drum. The third method will 
soon be tested underground on a standard 
drum-type continuous miner. 



REFERENCES 



1. Bobick, T. G. , and D. A. Giardino. 
Noise Environment of the Underground Coal 
Mine. MSHA Inf. Rep. 1034, 1976, 26 pp. 

2. Becker, R. S., and J. E. Robertson. 
An Investigation of Continuous Miner Coal 
Cutting Noise. Wyle Lab. Tech. Memoran- 
dum TM 80-6, Sept. 1980, 73 pp.; availa- 
ble upon request from M. R. Pettitt, Wyle 
Lab., Huntsville, AL. 

3. Becker, R. S., and G. R. Anderson 
II. An Investigation of the Mechanics 
and Noise Associated With Coal Cutting 
(contract J0177060, Wyle Lab). BuMines 



OFR 60-81, 1980, 275 pp.; NTIS PB 81- 
215394. 

4. Roepke, W. W. , and J. C. Church. 
Measuring In-Seam Coal Cutting Forces. 
Min. Eng. (N.Y.), v. 35, 1983, pp. 1281- 
1286. 

5. Becker, R. S., G. R. Anderson II, 
and T. E. Watts. An Investigation of 
the Mechanics and Noise Associated With 
In Situ Coal Cutting. Wyle Lab. Tech. 
Memorandum TM 81-13, Feb. 1981, 107 pp.; 
available upon request from M. R. Pet- 
titt, Wyle Lab., Huntsville, AL. 



65 



6. Anderson, G. R. II. Baseline 
Studies on the Feasibility of Detecting a 
Coal/ Shale Interface With a Self -Powered 
Sensitized Pick. Wyle Lab. Tech. Mem- 
orandum TM 81-3; available upon request 
from M. R. Pettitt, Wyle Lab., Hunts- 
ville, AL. 

7. Black, S., B. V. Johnson, R. L. 
Schmidt, B. Banerjee. Effect of Contin- 
uous Miner Parameters on the Generation 
of Respirable Dust. Min. Congr. J., v. 
64, No. 4, 1978, pp. 19-25. 

8. Black, S., and J. Rounds. Deep 
Cutting Continuous Miner. Effect of Drum 
Rotational Speed and Depth of Cut on Air- 
borne Respirable Dust and Specific Energy 
(contract H0122039, Ingersoll-Rand Res., 
Inc.). BuMines OFR 154-77, 1977, 288 
pp.; NTIS PB 274 345. 

9. Roepke, W. W. , D. P. Lindroth, and 
T. A. Myren. Reduction of Dust and Ener- 
gy During Coal Cutting Using Point Attack 



Bits, With an Analysis of Rotary Cutting 
and Development of a New Cutting Concept. 
BuMines RI 8185, 1976, 53 pp. 

10. Wyle Laboratories. Investigation 
and Control of Noise Generated During 
Coal Cutting. Ongoing BuMines contract 
S0387229; for inf., contact W. W. Aljoe, 
TPO, Pittsburgh Research Center, BuMines, 
Pittsburgh, PA. 

11. Pettitt, M. R. Development of a 
Reduced-Noise Auger Miner Cutting Head. 
Final report on BuMines contract H0188065 
with Wyle Lab., Mar. 1983; available upon 
request from W. W. Aljoe, Pittsburgh 
Research Center, BuMines, Pittsburgh, 
PA. 

12. Wyle Laboratories. Noise Control 
of Longwall Mining Systems. Ongoing Bu- 
Mines contract J0188072; for inf., con- 
tact W. W. Aljoe, TPO, Pittsburgh Re- 
search Center, BuMines, Pittsburgh, PA. 



66 



MANTRIP NOISE CONTROLS 
By Roy C. Bartholomae 1 and Thomas G. Bobick 2 



ABSTRACT 



The interior noise of an underground 
mine rail-operated personnel carrier 
(mantrip vehicle) was cost effectively 
reduced by replacing some standard com- 
ponents with acoustically treated com- 
ponents. The noise control features in- 
cluded a softer suspension, softer motor 
mounts , damped panels , sound-absorbing 
motor enclosures, and helical gears. 



Depending on operating conditions, the 
modified vehicle was 6 to 7.5 dBA quiet- 
er than an unquieted mantrip. The noise 
level in the mantrip interior was re- 
duced to approximately 85 dBA at an aver- 
age vehicle speed. These noise control 
features increased the overall mantrip 
cost by less than 5 pet. 



INTRODUCTION 



During a normal working shift, under- 
ground coal miners are exposed to a 
variety of noise sources , one of which is 
the rail personnel carrier, or mantrip 
vehicle, that transports them between the 
entrance and working sections of the 
mine. 

The interior noise level of most man- 
trips ranges between 90 and 100 dBA at 
typical operating speeds (6 to 14 mph) . 
Consequently, the mantrip vehicles con- 
tribute to the total daily noise dosage 
received by the workers. Although man- 
trips are less noisy and are used for 
shorter periods (30 to 60 min per shift) 
than other mining equipment, their 
cumulative contribution to noise exposure 
in mines is substantial because they af- 
fect a large number of workers. 

This paper describes a follow-on proj- 
ect of a previous retrofit program for an 
existing mantrip. This work extends and 
incorporates noise control treatments 
into the standard design of a new FMC 3 
mantrip model, with the goal of achieving 
an interior sound level of 85 dBA or less 



'Supervisory electrical engineer. 

^Mining engineer. 
Pittsburgh Research Center, Bureau of 
Mines, Pittsburgh, PA. 

3 Reference to specific products does 
not imply endorsement by the Bureau of 
Mines. 



at an average operating speed. A second 
goal was to design noise control treat- 
ments of general scope, so they could be 
incorporated into mantrips produced by 
other manufacturers. 

Untreated Mantrip 

Several kinds of mantrips are produced, 
primarily by five manufacturers. Most of 
the vehicles have closed tops and are 
trolley operated, but there are also some 
open-top and some battery-operated mod- 
els. Although drive trains and suspen- 
sion systems differ from one vehicle man- 
ufacturer to another, generally the basic 
structure of all mantrips is the same. 

The FMC model selected for this project 
has a closed top, is trolley operated, 
and has duplicate controls that allow 
operation from either end of the vehicle. 
The interior is divided into a middle 
(passenger) and two-end (operator) com- 
partments and is only accessible from one 
side of the vehicle (fig. 1). 

The vehicle body consists of a steel 
framework that supports the floor plates , 
sidewall panels, and roof panels. The 
chassis is suspended on two wheel sets, 
each consisting of flanged wheels on sol- 
id axles. The FMC suspension uses a 
trailing arm system. Each of the four 
suspension arms is pinned to the frame by 
a spherical bushing on one end and by a 



67 




FIGURE 1. - FMC model 2510 high-frame mantrip. 



coil spring-shock absorber combination on 
the other end. Two vertical guide plates 
restrain lateral arm movement. 

The drive system consists of electric 
motors, traction drive gear cases, and 
the axle wheel sets , usually with one 
motor per axle. The motors are located 
inside the passenger compartment next to 
the closed side of the vehicle. They are 
connected to the gear cases , which are 
outside the compartments, with a drive 
shaft through a hole on the sidewall. 
The drive train components are intercon- 
nected with universal joints. Service 
brakes are attached to the axle or to the 
gearbox output shaft. 

Depending on the model, the FMC man- 
trips transport 10 to 26 persons at a 
maximum speed of up to 17 mph. The man- 
trips are available in sizes ranging from 
15 to 24 ft in length, 6 to 8 ft in 
width, and 2 to 4.5 ft in height above 
the rail. 



Noise Sources 

Noise from the mantrip is generated 
by the wheel-rail system, the drive mo- 
tor, and the drive train. It reaches 
the vehicle interior through airborne 
and structureborne paths. These noise 
sources are shown in figure 2. 

The primary wheel-rail noise is struc- 
tureborne. It is transmitted into the 
main structure through the suspension arm 
bushing and the spring (fig. 2A) . An- 
other contribution results from lateral 
or vertical impacts at the suspension arm 
guide. The rail itself contributes air- 
borne noise. 

The motor (fig. 2B) contributes both 
airborne and structureborne noise; the 
former is emitted by the motor compo- 
nents , while the latter is radiated by 
the vehicle panels. 



68 



Frame 



KEY 



-*» Airborne path 

•*- Structureborne path 




Bushing 



Passenger / 

nirrnnhnnp— ' 



microphone 



Operator \ 
microphone -* 



PLAN VIEW 



Observer 



Motor 



1 



A. 



Floor 




FIGURE 2. - Mantrip noise sources and paths. 
A, Wheel-rail; B, motor; C, drive chain. 

Drive train noise is produced primarily 
by gear teeth engagement forces. Air- 
borne noise is radiated from the gear 
reducer housing. Vibration that gener- 
ates structureborne noise is transmitted 
to the structure through the motor and 
through the suspension arm (fig. 2C) . 

Additional sources, such as intermit- 
tent impacts from loosely supported pan- 
els, are of secondary importance. They 
are not addressed in this paper because 
they can be eliminated through proper 
maintenance. 

Testing Procedures and Results 

The contributions of the major airborne 
and structureborne noise sources were de- 
termined by a series of baseline and 
diagnostic tests. The baseline tests 
were conducted in an underground coal 
mine; and, since the reverberation ef- 
fects of tunnels are negligible in the 




Passenger Operator 
microphone-^ microphone 




v s 



ELEVATION 
FIGURE 3. - Mantrip noise measurement locations. 

interior of closed-top mantrips, 4 the 
diagnostic tests were conducted in the 
manufacturer's assembly plant, where 
conditions were well controlled. The 
noise was measured in the middle compart- 
ment and in one end compartment (fig. 
3) , referred to as passenger and operator 
compartments, respectively. 

Ideally, all tests should have been 
performed on mantrips of the same model. 
This was impossible, however, and the un- 
derground baseline tests and the above- 
ground diagnostic tests were performed on 
untreated FMC 2870 and an untreated FMC 
2510 models, respectively. That is, for 
the noise source diagnosis and selection 
of treatments , it was assumed that the 
noise characteristics of most equal size 
and weight FMC mantrips are similar. 
Howevever, this assumption, which is dis- 
cussed further in a later section, had no 
impact on the final evaluation of the 

4 Galaitsis, A. G., P. J. Remington, and 
M. M. Myles. Noise Control of a Mine Op- 
erated Rail Personnel Carrier. Volume I. 
Design and Performance of Noise Control 
Treatments (contract J01 66090, Bolt 
Beranek and Newman Inc.). BuMines OFR 
133-78, 1977, 116 pp.; NTIS PB 289 711. 



69 



treatments, which were performed by di- 
rectly comparing the underground noise of 
two mantrips , one treated and one un- 
treated, of the same model. 

Typical underground noise spectra mea- 
sured in the operator and passenger com- 
partments of an FMC 2870 mantrip are 
shown in figure 4. It was observed that 
the mantrip noise increased with speed. 
Measurements taken between 8 and 14 mph 
showed that the A-weighted sound level 
increased by approximately 0.7 dBA per 
each 1-mph speed increase. The data 
shown in figure 4 were obtained at 10 
mph, the average speed in the mine that 
purchased the treated mantrip. 

The aboveground diagnostic tests iden- 
tified different source contributions 
by suppressing certain other sources. 
Various methods were used, including 
temporary acoustical treatments, disen- 
gagement of drive train components , and 
artificially created operating condi- 
tions. For example, the wheel-rail noise 
was eliminated by operating the vehicle 
on jacks; similarly, the drive train 
noise was eliminated by disengaging 
the motor from the gear reducer. Addi- 
tional details on the diagnostic tests 
and the spectral composition of noise 
contributions are found in Ferrari and 
Galaitsis.^ 

The diagnostic tests were performed at 
12.8 mph, a speed that could be main- 
tained constant for a sufficiently long 
period of time. At this speed, the con- 
tributions of the major noise sources at 
the passenger's compartment were as fol- 
lows, in decibels (A-weighted): 

Wheel-rail .... 94 
Motor 88 

Drive train. . . 83 

The wheel-rail noise and the motor 
noise each exceeded the 85-dBA goal, and 

^Ferrari, V., and A. Galaitsis. Inte- 
gration of Quieting Technology Into New 
Mantrip Vehicles (contract J01 99068, ESD 
Corp.). BuMines OFR 62-82, 1981, 164 
pp.; NTIS PB 82-203241. 



~~ ' 1 ' 1 

Untreated underground 




2,000 8,000 32,000 

ONE-THIRD OCTAVE BAND CENTER 
FREQUENCY, Hz 

FIGURE 4. - Typical mantrip noise in different 
compartments, at a 10-mph speed. 

therefore treatment of the associated 
noise sources and/or paths was mandatory. 
The drive train was also treated to lower 
noise reduction requirements for the 
wheel-rail and motor noise contribution. 

Acoustical Treatments 

Initially, the list of potential noise 
control treatments for the mantrip in- 
cluded resilient wheels, damped wheels, 
self -steering truck, 6 constrained layer 
damping, sound-absorbing panels, isolated 
suspension spring seat, isolated suspen- 
sion shock mount bushing, isolated sus- 
pension arm bushing, suspension arm guide 
plate isolation, resilient motor mounts, 
tightly sealed and sound-absorbing motor 
enclosures, helical gears, and constant- 
velocity U-joints. After a cost-benefit 
analysis, performed by the manufacturer, 
the following noise control treatments 
were selected for installation on an FMC 
2450 mantrip: panel damping, soft spring 
seats , soft suspension arm bushings , sus- 
pension arm guide plate isolators , motor 
enclosures , motor mounts , and helical 
gears. 

6 List, H. A., W. N. Caldwell, and 
P. Marcotte. Proposed Solutions to the 
Freight Car Truck Problems of Flange 
Wear and Truck Hunting. ASME paper 75- 
WA/RT-8, 1975, 7 pp. 

Scheffel, H. Self -Steering Wheelsets 
Will Reduce Wear and Permit Higher 
Speeds. Railw. Gaz . Int., v. 132, No. 
12, 1976, pp. 453-456. 



70 



The composite loss factor of the stan- 
dard 1/8-in steel walls and ceiling 
of the untreated mantrip was between 
0.003 and 0.02. Approximately 70 pet of 
these panels were replaced by damped 
NEXDAMP — II sheets , which resulted in a 
composite loss factor between 0.02 and 
0.1.7 The NEXDAMP-II, manufactured by 
U.S. Steel, is a three-layer (steel- 
viscoelastomersteel) laminate available 
in various thicknesses. Sheets of 0.148- 
in-thick NEXDAMP-II, consisting of a 
0.020-in viscoelastic layer sandwiched 
between two 0.064-in steel layers were 
selected for the current application. 

The suspension system modifications 
were designed to reduce the wheel-rail 
structureborne noise. The treatments 
consisted of resilient components intro- 
duced at the three contact areas between 
each suspension arm and the main struc- 
ture, that is, at the bushing, spring, 

. 

'Work cited in footnote 5. 



and guide plates (fig. 5). Specifically, 
two 1/4-in rubber sleeves and two 3/8-in 
washer-shaped rubber seats (all 55- 
durometer) were used per spring; the 
standard metal bushing was replaced by 
a 2-1/8-in-ID, 3-1/2-in-OD, 55-durometer 
rubber bushing; strips of 1/4-in Linerite 
abrasion-resistant polymer backed by 3/8- 
in rubber were inserted between the sus- 
pension arm tip and its guide plates. 

In the standard configuration, the mo- 
tors, which are located in the passenger 
compartment, are safeguarded by partial 
metal covers. The motor airborne noise 
was reduced by replacing these covers 
with tight fitting ones and by lining 
the walls and ceiling of the resulting 
enclosure with sound-absorbing material 
(fig. 6). The sound-absorbing liner was 
1-in-thick Owens Corning fiberglass type 
705, attached to the enclosure walls by 
bendable-tip acoustical material fasten- 
ers. Proper ventilation was maintained 
through the sidewall opening (between the 



Rubber seat 



Rubber bushing 



Suspension arm 



Guide plate 




Steel liner 



Rubber 
sleeve 



Linerite 




Metal 
sleeve 




Rubber washer 



Rubber sheet 



SECTION A-A 



SECTION B-B 



SECTION C-C 




FIGURE 5. - Major features of modified suspension components. 



71 



I- in-thick fiberglass insulation on top 
and sides of motor compartment 




Rubber mount 

FIGURE 6. - Treatments for the reduction of motor noise. 



enclosure and the exterior of the vehi- 
cle) that accommodates the drive shaft. 
The motor structureborne noise was re- 
duced by attaching the motor to the frame 
with Barry Industries type G05-04 mounts. 
Finally, the drive train noise of the 
modified mantrip was reduced by re- 
placing the standard spur gears with 
helical gears. 

The basic materials for the var- 
ious treatments (NEXDAMP-II, Fiberglas, 
Linerite, etc.) are commercially availa- 
ble, but they required some cutting or 
shaping prior to installation. The only 
items that were specially made, but are 
now FMC stock items , were the suspension 
rubber bushings and the helical gears. 

Effectiveness of Noise Controls 

The effectiveness of the noise control 
treatments was determined by comparing 
the sound levels of a treated and an un- 
treated FMC 2450 mantrip, in the same 



underground mine under similar operating 
conditions. The results are summarized 
in figures 7 through 9. 

Figures 7 and 8 show typical time his- 
tories and noise spectra of the two 
vehicles at 10 mph. Both figures indi- 
cate that the selected treatments result- 
ed in a significant noise reduction. 

Figure 7 shows that the short-time- 
average noise level fluctuates even at a 
constant speed. This variation stems 
from uneven track conditions associated 
with rail joints, rail wear state, and 
track slope. The time traces correspond 
to simultaneous recordings in the two 
compartments during inbound runs . The 
two vehicles were tested at different 
times , but within the same day and over 
approximately the same track section. 

Typical noise spectra in both com- 
partments of the treated and untreated 
vehicles are compared in figure 8; they 



72 



00 



m 


90 


TJ 




r» 




_l 




UJ 




> 

UJ 


80 


_1 




Q 




7* 




ZD 


fQ 


O 




en 


100 


Q 




UJ 




h- 




X 


90 


<s> 




UJ 





Untreated 




Passenger 



Treated- 

i L 




80 



70 



-Untreated 




Operator 



Treated ■ 







10 



30 



20 

TIME, s 

FIGURE 7. - Typical noise-time histories in 
passenger and operator compartments of untreat- 
ed and treated mantrips, at a 10-mph speed. 



KEY 

Passenger Operator 

• Untreated 91.5 dB(A) * Untreated 91 dB(A) 
° Treated 84dB(A) * Treated 84dB(A) 



40 




iu " 31.5 125 500 2,000 8,000 32,000 

% £ ONE-THIRD OCTAVE BAND CENTER FREQUENCY, Hz 

FIGURE 8. - Typical noise spectra in treated 
and untreated mantrips, at a 10-mph speed. 

correspond to 4-s samples selected ran- 
domly from the 40-s-long traces of figure 
7. The combined noise reduction from all 
treatments is maximum between 125 and 
2,000 Hz, where the untreated vehicle 
noise is dominant. 

Figure 9 shows the vehicle noise depen- 
dence on speed. Multiple measurements 
were performed at each speed over differ- 
ent track sections to estimate the typi- 
cal data spread (shaded areas) resulting 



00 

LU 
> 

UJ 

_l 

Q 



00 



90 



Z) 


80 


o 

CO 


100 


Q 




UJ 




K 




X 




o 




UJ 


90 


< 





FMC 2450 
operator 

Untreated 




1 r 

FMC 2450 
passenger 




Untreated- 




Treated 



8 10 12 14 
VEHICLE SPEED, mph 

FIGURE 9. - Dependence of mantrip noise on speed. 



from uneven track conditions. The 
straight lines represent the fit of a 
least squares curve through each group of 
points, and they may be used to estimate 
the average noise within the range of 
measured speeds. 

Figures 4 and 8 also shed some light on 
the validity of the assumption that all 
FMC mantrips generate similar noise. 
Clearly, there is a general resemblance 
(major peak at 315 to 400 Hz) between the 



73 



traces of the untreated FMC 2870 (fig. 4) 
and FMC 2450 (fig. 8) models, correspond- 
ing to the same compartments. There are 
also noticeable differences (lack of 80- 
Hz peak for the operator of the model 
2870); however, such differences are to 
be expected in view of the noise varia- 
tions recorded for a single mantrip (fig. 
9); therefore, the general-similarity 
assumption made during the diagnostic 
stages of the study was justifiable. 

Prolonged observations during the un- 
derground measurements showed that the 
average operating speed in the mine that 



owned the treated vehicle was about 10 
mph. Therefore, personal noise exposure 
from the treated mantrip should be de- 
scribed in terms of the 10-mph sound 
pressure levels. Figure 8 shows that at 
a vehicle speed of 10 mph, the sound lev- 
els inside the operator and passenger 
compartments were reduced from approxi- 
mately 91 dBA to 84 dBA. Clearly, the 
85-dBA goal has been achieved only for 
FMC 2450 mantrips operated at an average 
speed of 10 mph or less ; typical noise 
levels for different average operating 
speeds may be obtained from figure 9. 



CONCLUSION 



The selected noise control treatments 
met the objective of an interior vehicle 
noise level of less than 85 dBA under 
average operating conditions. In the 
opinion of the workers using the vehicle, 
these treatments also improved the ve- 
hicle riding comfort; this benefit re- 
sulted primarily from the compliant bush- 
ings, which improved the isolation be- 
tween the vehicle body and the wheels. 



After 18 months of underground vehicle 
service, none of the treatments has shown 
signs of wear; therefore, their durabil- 
ity is satisfactory. According to the 
manufacturer, the modifications raised 
the cost of a new mantrip by 4.3 pet. 
The treatments can also be installed on 
most existing FMC models on a retrofit 
basis during equipment overhaul. 



74 



QUIETED PERCUSSION DRILLS 
By William W. Aljoe 1 



ABSTRACT 



Percussion-type rock drills are common- 
ly used in both coal and metal-nonmetal 
mines; they produce extremely high noise 
levels (110 to 120 dBA) and have complex 
noise-generating mechanisms. Therefore, 
engineering noise controls for percus- 
sion drills have been very difficult to 



achieve. However, substantial progress 
has been made toward this goal through 
the use of retrofit techniques and per- 
cussion drill redesign. This paper pro- 
vides an overview of Bureau-sponsored re- 
search programs aimed at reducing the 
noise produced by percussion drills. 



INTRODUCTION 



Percussion-type rock drills, especially 
pneumatic drills, are the noisiest ma- 
chines used on a regular basis by the 
mining industry. Handheld and machine- 
mounted "jumbo" percussion drills are the 
most common means of drilling production 
bias tholes and roof bolt holes in under- 
ground metal and nonmetal mines. Hand- 
held "stoper" drills are also used in 
coal mines; although their use has de- 
creased in recent years , they are still 
used for "spot" bolting in roof fall 
areas and other mine locations where 
machine-mounted rotary roof bolters can- 
not reach. Typical noise levels of- un- 
muffled percussion drills range from 110 
to 120 dBA at the operator's position, 
potentially resulting in exposures of 
more than 10 times the limits allowed un- 
der Federal regulations. For this rea- 
son, the Bureau of Mines has directed 
substantial research efforts toward the 
control of percussion drill noise. 

NOISE SOURCES AND ABATEMENT TECHNIQUES 

Hand-Held Drills 

Figure 1 shows the three most prominent 
noise sources on a typical pneumatic 
stoper drill: (1) drill steel vibration, 
(2) drill body vibration, and (3) air ex- 
haust. Although exhaust noise is often 
the dominant source, drill body and drill 
steel vibration frequently contribute 



Mining engineer, Pittsburgh Research 



greatly to the total drill noise level. 
Therefore, stoper drill noise has a wide 
spectral distribution, and all three 
noise sources shown in figure 1 must be 
controlled to achieve a substantial noise 
reduction. 

Because exhaust noise levels alone can 
be as high as 112 to 114 dBA (1_), 2 an ex- 
haust muffler is the first and most im- 
portant component of any noise control 
treatment for pneumatic drills. Mufflers 
for the hand-held pneumatic drills used 
in mining have usually taken one of two 
forms: (1) a canister, lined duct, or 
other chamber-type device attached to the 
drill exhaust port; or (2) a wraparound, 
jacket-type muffler that completely sur- 
rounds all or part of the drill body, 
including the exhaust port. The second 
type of muffler, if designed and con- 
structed properly, can reduce drill body 
noise as well as exhaust noise. However, 
both types of mufflers share a common 
problem — freezing. This occurs when 
moisture in the rapidly expanding, rapid- 
ly cooling exhaust air condenses and 
freezes on the inside surfaces of the 
muffler and drill body. After a short 
period of time, perhaps only a few min- 
utes, the ice buildup restricts the flow 
of exhaust air, and the resulting back 
pressure causes the drill to stall. 

2 Underlined numbers in parentheses re- 
fer to items in the list of references 
at the end of this paper. 



Center, Bureau of Mines, Pittsburgh, PA. 



75 



Drill steel 




FIGURE 1. - Noise sources of typical hand* 
held stoper drill. 



Numerous muffler designs for hand-held 
mining drills have been developed by 
equipment manufacturers , mining compa- 
nies , and government researchers . Many 
of these designs are reviewed in a recent 
Bureau-sponsored report by Dutta and Run- 
stadler (2^) and in various MSHA publica- 
tions (3) . Muffling schemes as simple as 
adding a section of rubber tire to the 
drill exhaust port have resulted in noise 
reductions as great as 9 dBA. However, 
the noise levels of these muffled drills 
were still unacceptably high (greater 
than 110 dBA). Therefore, as described 
later in this paper, the Bureau sponsored 
a research program to redesign the stan- 
dard hand-held mining drill for noise 
control purposes. 

Drill steel vibration alone produces 
noise levels of about 105 to 110 dBA at 
the operator's position ( 4-5 ) , mostly be- 
cause of transverse stress waves within 
the steel. In contrast to longitudinal 
stress waves, which effectively transmit 
percussive energy from the drill to the 
rock, transverse waves serve no useful 
purpose and merely generate noise. Ex- 
tensive tests have shown that trans- 
verse waves result mainly from off cen- 
ter (e.g., piston-to-steel) impacts, worn 
drill chucks, and bent drill steels. 

Better fitting, longer lasting drill 
components would obviously reduce the 
severity of drill steel (and drill body) 
noise but could not eliminate it because 
of the high-energy nature of the percus- 
sive process. Aside from drill manufac- 
turers' efforts to produce more effi- 
cient drill hardware, only a few studies 
have investigated methods to reduce drill 
steel and drill body noise. Visnapuu and 
Jensen (1_) developed a constrained-layer 
damping system for the drill steel, which 
consisted of a thin-walled tubular metal 
collar bonded to the drill steel by vis- 
coelastic material. This "sheathed" 
steel was prepared by slipping the metal 
tube over the steel and pouring the liq- 
uid viscoelastic filler into the annulus. 
A similar system was used by Summers 
and Murphy (6) to produce a 6-in-long 
isolation-damping collar for the end of 
the steel closest to the drill body. 



76 



Although drill steel coating and col- 
laring techniques such as those described 
can reduce noise, studies by Hawkes (4-5) 
showed that they can reduce the drilling 
rate significantly, are subject to abra- 
sion, and can move axially along the 
steel because of failure of the visco- 
elastic bond. However, Hawkes concluded 
that an independent "shroud tube" sur- 
rounding the steel would be able to 
suppress drill steel noise while avoid- 
ing these problems. The redesigned coal 
stoper discussed later in this paper uti- 
lized the "independent" shroud tube con- 
cept suggested by Hawkes. 



because air exhaust noise 
However, hydraulic drills 
"quiet," because drill s 
body vibration, and noi 
hole-flushing air combine 
ical noise levels of 11 
the operator's position. 
Bureau is currently invest 
noise control techniques 
percussion drills. 



is nonexistent, 
are by no means 
teel vibration, 
se produced by 
to produce typ- 
to 113 dBA at 
Therefore, the 
igating various 
for hydraulic 



BUREAU-DEVELOPED NOISE-CONTROL 
TECHNIQUES FOR HAND-HELD DRILLS 

Stoper Retrofit Treatment s 



Machine-Mounted "Jumbo" Drills 

Figure 2 shows the major components of 
a typical jumbo drill rig, and figure 3 
shows its noise-producing mechanisms. As 
with hand-held drills, the three major 
noise sources of pneumatic jumbo drills 
are the air exhaust, drill steel, and 
drill body, and the two most essential 
noise control treatments are an exhaust 
muffler and an enclosure around the drill 
steel. Exhaust muffling techniques for 
jumbo drills have included (1) piping it 
away from the operator through ductwork, 
(2) attaching a canister-type muffler to 
the exhaust port, and (3) placing the en- 
tire drifter inside an acoustical en- 
closure. To date, it appears that the 
acoustical enclosure technique has been 
the most effective because it reduces 
drill body noise as well as exhaust 
noise, and is the least susceptible to 
freezing. Drill steel coating, rubber 
collars, and shroud tubes have been used 
on jumbo drills to suppress drill steel 
noise; again, the shroud tube appears 
to be the most promising technique be- 
cause it is not physically coupled to the 
drill steel. Bureau-sponsored research 
on acoustical enclosures and shroud tubes 
for pneumatic jumbo drills is described 
in detail later in this paper. 

In recent years, hydraulically powered 
jumbo drills have become very popular 
in the mining industry. In terms of 
noise control, hydraulic drills have a 
distinct advantage over pneumatic drills 



Figure 4 shows the stoper drill retro- 
fit treatment developed by Summers and 
Murphy ( ,5_) . The wraparound, jacket-type 
muffler was made of a flexible sheet of 
polymer material. Poured-in urethane end 
caps held the jacket muffler in place and 
isolated it from drill body vibration. 
The drill steel was treated with the 
6-in-long composite damping collar de- 
scribed earlier. 

In underground tests at the Bureau's 
Pittsburgh (PA) Research Center, an un- 
treated stoper (fig. 1) produced noise 
levels of about 115 dBA. Addition of the 
jacket muffler resulted in a 13-dBA noise 
reduction, and the drill steel collar 
produced an additional 2-dBA reduction. 
The quieted noise level of 100 dBA would 
permit about 2 h of operating time per 
shift (versus zero with the untreated 
stoper) without violating Federal noise 
regulations. The retrofit package in- 
creased the total drill weight by about 
10 lb. 

Retrofitted stopers (jacket muffler 
only) were tested in 15 operating under- 
ground coal mines, and noise reductions 
of 7 to 8 dBA were consistently obtained. 
The 13-dBA experimental noise reduction 
was not achieved in the underground mine 
tests. The drill feed rate was not con- 
trolled nor the noise control treatments 
maintained as diligently as they were in 
the Bureau's experimental mine. Never- 
theless, the typical muffled noise lev- 
els (105 to 106 dBA) were low enough to 



77 



permit a doubling of the allowable oper- were about 15 to 50 pet slower than with 
ating time per shift. Unfortunately, an unmuffled stoper. 
drilling rates with the modified stoper 



Jumbo 



Drifter 



Steel 



Bit 



L 




/ 



^e=^ 



Feed 




^ 



Centralizer 



25 ft 



FIGURE 2. - Major components of jumbo drilling rig. 



Piston-striking bar impact 

Leakage air noise 

Air motor noise 

Body noise 

J"" Exhaust noise 




Bit-rock impact 



FIGURE 3. - Noise sources of jumbo-mounted drills. 



78 




FIGURE 4. - Stoper drill with retrofit noise control treatments. 



79 



Stoper Redesign 

Because the stoper retrofit noise con- 
trol treatments were only partially 
successful, the Bureau sought greater 
noise reductions and improved drilling 
performance through stoper redesign. 
The Bureau-sponsored redesign effort (2^) 
included the following four major steps: 

(1) redesign of the drill steel rota- 
tion mechanism and other drill parts, 

(2) development of a compact, effective 
muffler-enclosure device, (3) development 
of a shroud tube to attenuate drill steel 
noise, and (4) redesign of all drilling 
controls. The redesigned stoper was then 
field tested in several operating under- 
ground coal mines. 

Redesign of Drill Steel Rotation 
System and Internal Parts 

Standard stoper drills achieve drill 
steel rotation through a "rifle bar" ar- 
rangement (see figure 5) . The oscillat- 
ing piston strikes the drill steel on its 
backstroke, a fluted hole in the center 
of the piston rides over the similarly 
fluted rifle bar, thus causing the pis- 
ton, chuck, and drill steel to rotate. 
The pawl-and-ratchet ring at the back of 



Rock 
Bit 

Drill rod 



Chuck 
Shank 

Piston 



Air port 



Downstroke chamber 
Valve 




Collar 

Chuck driver nut 
Drill body 

Return-stroke chamber 

Exhaust port 
Rifle bar 
Compressed air 

Pawl, ratchet 
Compressed air 



Airleg 



FIGURE 5. - Components of typical stoper 
with rifle-bar rotation. 



the drill cylinder maintains drill steel 
rotation in only one direction. Because 
the piston stroke length and helical an- 
gles of fluting are all fixed for any 
given drill, the rotation torque is con- 
stant. The rotation speed is therefore 
dependent on the thrust level provided by 
the drill feed leg. 

The redesigned stoper (fig. 6) utilized 
an independent drill steel rotation sys- 
tem (i.e., a separate air motor and gear 
arrangement) that gave it several dis- 
tinct advantages over drills with rifle- 
bar rotation. First, drill performance 
was improved because the rotation speed 
was no longer dependent on thrust. That 
is, the poor penetration rates associated 
with over-rotation (not enough thrust) 
and drill stalling problems due to under- 
rotation (too much thrust) were elimi- 
nated. The piston was always able to 
travel through its full stroke , thus in- 
creasing drilling power, and rotation 
speed could be changed to suit differ- 
ent rock conditions without affecting 
the piston blow frequency. Second, the 
multiple internal impact points of the 
rifle-bar system were eliminated, thus 
reducing the high-frequency rattling 
noise produced by standard stoper drills. 
Third, the piston diameter was reduced, 
without sacrificing drilling power, by 
eliminating the need for the fluted 
hole in its center. This resulted in 
a smaller overall drill diameter and 
facilitated the subsequent addition of 
the muffler-enclosure device. 

The independent drill steel rotation 
system allowed several other beneficial 
internal design changes to be made. Be- 
cause the piston was no longer responsi- 
ble for rotation, it was redesigned to 
serve as the valve controlling the flow 
of . compressed air within the drill cylin- 
der. This "valveless" method of piston 
operation was inherently more efficient 
and problem-free than a valve system; 
in addition, the elimination of the stan- 
dard "flapper" and "kicker-port" valves 
negated another potential source of high- 
frequency noise. The annular clearance 
between the chuck and shank was reduced 
to 0.02 in, and the upset shoulder on 



80 



^^ 



Baffle plate 
(ear- isodamp) 



Piston 







^^^ 



Dust exhaust ^-/) 



Back head 



Deflector plate 
(ear-isodamp) 




Rotation air 
motor 



Exhaust shroud tube 
isolation sleeve 



^^F 



// / / / / 






i i i t i =t 




Muffler outer 

layer 
(ear-isodamp) 



Muffler inner 

layer 
(aluminum) 



Front head 
(ear-isodamp) 



FIGURE 6. - Internal components of redesigned, "quiet" stoper. 



the standard drill steel was eliminated. 
These two design changes reduced the 
misalignment and rattling impacts occur- 
ring at the top of the drill body -and re- 
duced the severity of the transverse 
waves produced by offcenter shank-to- 
steel impacts. 

Muffler-Enclosure Device 

The new drill body design necessitated 
the development of the special muffler- 
enclosure device shown in figure 6. The 
inner part of the muffler-enclosure con- 
sisted of a series of ring-shaped, per- 
forated metal baffle plates around the 
drill body. The outer shell of the en- 
closure consisted of two layers — an inner 
aluminum layer and an outer layer made of 
EAR Isodamp 1002 3 polymer material. Dur- 
ing drilling, the exhaust air from the 
piston chamber and rotation motor moved 
upward through the perforated baffle 

■^Reference to specific products does 
not imply endorsement by the Bureau of 
Mines. 



plates and left the acoustical enclosure 
through an exhaust hole near the top of 
the drill. 

The muffler-enclosure attenuated both 
drill body noise and air exhaust noise, 
and the baffle plates vibrated to in- 
hibit ice buildup on the inner surfaces 
of the enclosure. A flexible deflector 
plate near the exhaust port of the pis- 
ton chamber directed the air toward the 
top of the drill and also helped reduce 
icing. In extended laboratory tests us- 
ing compressed air saturated with wa- 
ter vapor, icing problems were virtually 
nonexistent. 

Shroud Tube 

The shroud tube of the redesigned 
stoper drill was a simple steel tube de- 
signed to fit a 1-in hexagonal drill 
steel and 1-3/8- to 1-3/4-in drill bits. 
Its outer diameter was small enough to 
allow it to follow the drill bit into the 
hole, and its inner diameter was large 
enough to keep it from touching the drill 



81 



steel (1/16-in annular clearance). The 
tube was connected to the top of the 
drill body through a rubber sleeve (see 
figure 6) to provide isolation from drill 
body vibration. 

Redesign of Drilling Controls 

The drilling controls of the redesigned 
stoper had several unique, advantageous 
features. All controls were mounted on 
the feed leg such that the operator could 
stand 2-1/2 ft away from the machine 
(versus 1 ft on a conventional stoper) , 
thereby exposing him or her to less 
noise. Hammer, thrust, and rotation con- 
trols were located together for easy 
operation. 

During drilling, the operator first 
moved the primary drill throttle handle 
to a special "collaring" position. This 
supplied pressure to the feed leg and 
started a light hammering action without 
rotation. After the bit had made suffi- 
cient penetration to assure a straight 
hole, the operator moved the throttle to 
the "full on" position (hammer, feed, and 
rotation) . A separate control could be 
used to alter rotation speed as neces- 
sary, and a special valving spindle al- 
lowed automatic reduction of stalling 
torque under high feed leg pressure. 
When the hole was completed, the operator 
moved the throttle to a special "drill 
retract" position. In this position, the 
hammering action ceased, and the feed 
pressure decreased to allow the feed leg 
to collapse; however, rotation continued 
and the drill bit augered its way smooth- 
ly out of the hole. 

Underground Tests of Redesigned Stopers 

Six of the prototype quiet stopers were 
manufactured, and four were tested in 
operating underground coal mines. The 
average noise level of the "quiet" stop- 
ers was about 102 dBA in these mines, 
approximately a 15-dBA reduction versus 
standard stopers. Importantly, the re- 
designed drills were lighter and their 
penetration rates were faster than the 
stopers they replaced, thus increasing 
their acceptance by the miners. Freezing 



problems did not occur, and only minimal 
wear was noted when the drill parts were 
examined at the conclusion of the field 
tests. The success of the redesigned 
drill was demonstrated by the fact that 
three of the four test mines offered to 
buy the drills when they became commer- 
cially available. The only disadvantage 
of the redesigned stoper was that the 
shroud tube required removal and replace- 
ment during the drill steel changing 
process. Since this proved to be time- 
consuming, operators often drilled with- 
out the shroud tube, partially negating 
the effectiveness of the redesigned 
drill. However, noise levels without the 
shroud tube were still about 107 dBA, 
substantially lower than standard stopers 
or stopers with retrofit mufflers. 

Redesign of Hand-Held Hardrock Drill 

Because of the success of the rede- 
signed coal stoper, the Bureau has spon- 
sored a program (7) to redesign a hand- 
held drill suitable for use in hard-rock 
mines. The basic design features of 
the "quiet" hard-rock drill (fig. 7) are 
the same as those of the "quiet" coal 
stoper — independent rotation, valveless 
operation, muffler-enclosure, shroud 
tube, and redesigned controls. However, 
the size, shape, and stroke length of the 
piston of the quiet coal stoper had to 
be changed substantially to achieve the 
higher blow energy needed in the hard- 
rock version of the drill. Computer mod- 
eling of the drills' percussion cycles 
(piston positions, porting arrangements, 
air pressures, etc.) greatly facilitated 
this process. 

The quiet hand-held hard-rock drill 
differed from the quiet coal stoper in 
several other ways (compare figures 6 and 
7). First, the outer cover of the hard- 
rock drill was made of cast aluminum 
rather than the aluminum-EAR composite; 
this reduced its weight and made it eas- 
ier to fabricate. Second, the ring- 
shaped baffle plates were eliminated 
in the hard-rock version of the drill 
because the flexible exhaust deflector 
alone was found to be sufficient to 
inhibit ice buildup. Third, the drill 



82 



Aluminum 
outer 
cover 
Front 1 

exhaust 



Isolation 
mounts 




Replaceable 
chuck Independent 

rotation 



Valveless 
hammer 



Flexible liner 
to prevent 

ice buildup 

FIGURE 7. - Internal components of quiet hand-held hardrock drill. 



cylinder was mounted within the outer 
cover through rubber pads that isolated 
the cover from drill cylinder vibration. 
These design changes resulted in a power- 
ful, quiet, lightweight hard-rock drill. 

In addition, the "quiet" hard-rock 
drill had to be able to drill horizontal 
and angled production holes as well as 
vertical roof bolt holes. Therefore, the 
feed leg mounting mechanism and control 
arrangement of the coal stoper were rede- 
signed for this purpose. The hammer and 
drill cylinder designs were also modified 
slightly to improve drill startup while 
in the horizontal position. A fiberglass 
feed leg was utilized to reduce overall 
drill weight. 

A production-ready prototype of the 
redesigned, "quiet" hard-rock drill pro- 
duced noise levels of 104 dBA (with 
shroud tube) to 107 dBA (without shroud 
tube) when tested in an underground hard- 
rock mine. Both the hard-rock and the 
coal versions of the "quiet" hand-held 
drill are now commercially available 
through Technological Enterprises Inc. 
(TEI), Littleton, CO. TEI reports that 
the new drills cost about the same as 



standard (unmuffled) drills, and provide 
better drill control features. Other 
commercial drill manufacturers can obtain 
complete design details from the Bureau. 

NOISE CONTROL TECHNIQUES 
FOR JUMBO DRILLS 

As with hand-held percussion drills, 
the Bureau has investigated both retrofit 
and redesign measures for reducing jumbo 
drill noise. A potentially workable ret- 
rofit package was developed under Bureau 
contract (8) and is now being tested in- 
house to determine its long-term durabil- 
ity. The redesigned jumbo drill is now 
being field-tested by another contractor 
(9) . As with stoper drills , the two ma- 
jor components of the noise-controlled 
jumbo drills were (1) a muffler-enclosure 
to attenuate air exhaust and drill body 
noise and (2) a shroud tube to attenuate 
drill steel noise. 

Retrofit Treatments 

Figure 8 shows the retrofit muffler- 
enclosure on a drifter with rifle-bar 
rotation. The muffler-enclosure had to 
surround the drifter completely because 



83 




FIGURE 8. - Drifter within retrofit muffler-enclosure (cover open). 



there were three air exhaust ports at 
different locations around the drill 
body. The halves of this two-piece, box- 
like enclosure fit together snugly around 
a horizontal centerline. Its octagon- 
shaped profile was a compromise reached 
after considering the requirements of in- 
terior volume , exterior slimness , noise- 
attenuating properties, and light weight. 
Figure 8 shows that the top portion of 
the enclosure was hinged to the bottom 
portion to allow easy access to the 
drill. 

The schematic drawing of the muffler- 
enclosure (fig. 9) shows that the exhaust 
air exited the drill radially, struck the 
silicone rubber deflector at the top of 
the enclosure , and moved forward to es- 
cape through the front opening. Because 
the deflector was very flexible, it shook 
off any ice that began to form on it. 
After passing the deflector, the exhaust 



air entered the fiberglass-lined muffler 
section at the front of the enclosure. 
(This muffler section can also be seen in 
figure 8.) A perforated metal plate held 
the fiberglass in place and a thin layer 
of Kapton film prevented it from absorb- 
ing oil and water. The exhaust air then 
left the enclosure through the opening at 
its front end. The three major advan- 
tages of this muffler-enclosure design 
were (1) exhaust noise was directed away 
from the operator and absorbed; (2) the 
cold exhaust air cooled the coupling and 
shank at the front of the drifter; 
and (3) the warm drill components heated 
the exhaust air, thus inhibiting ice 
formation. 

Figure 10 shows the components of the 
shroud tube surrounding the drill steel. 
The outer diameter of the shroud tube was 
slightly smaller than the bit diameter, 
allowing the tube to enter the hole 



84 



Perforated plate 
muffler section 



Enclost 



Muffler section 
(fiberglass retained by 
perforated plate) - 



TTT 

-Exhaust airflow 




Tapered exhaust exit 

transition 




Z-bar clamp 



"Drill coupler , 



_/ 






Muffler section 



Enclosure 



Feed channel 

FIGURE 9. - Schematic view of retrofit drifter enclosure. 



Drifter 
enclosure 



Drill steel 
Plastic liner- 
Foam interlayer/ 
Steel tube. 





"Coupling cover 
'Coupling 

FIGURE 10. • Schematic view of retrofit drill steel 
shroud tube. 

behind the bit. The inner polymer layer 
rode loosely on the drill steel, causing 



the tube to rotate slightly during opera- 
tion. The foam interlayer absorbed some 
of the vibration imparted to the polymer, 
and the steel outer layer protected the 
two inner layers from damage. Exhaust 
air from the muffler-enclosure traveled 
forward through the annulus between the 
steel and the shroud tube, escaping just 
behind the bit. 

Performance of the jumbo drill with 
and without the retrofit noise control 
treatments was evaluated first in the 
laboratory, then at an above-ground test 
site. Laboratory tests in a reverbera- 
tion room (fig. 8) showed that the sound 
power level of the treated drill was 19.3 
dBA lower than that of the untreated 
drill. At the aboveground test site, 
noise reductions of 16.5 to 18.5 dBA were 
recorded at the operator's position (ta- 
ble 1). Diagnostic tests showed that the 



TABLE 1. - Acoustical performance of retrofit jumbo drill noise control 
treatments, A-weighted overall noise level, decibels 



Drill position 


Base- 
line 


Fully 
quieted 


Reduc- 
tion 


Drill position 


Base- 
line 


Fully 
quieted 


Reduc- 
tion 


ABOVEGROUND TESTS 


UNDERGROUND TESTS 


Collaring hole, 
10 ft of steel.. 

Middle of hole, 5 
to 6 ft of steel 

End of hole, 1 to 
2 ft of steel... 


108.5 

107 
105.5 


92 

88.5 

88 


16.5 
18.5 
17.5 


Collaring hole, 
10 ft of steel.. 

Middle of hole, 5 
to 6 ft of steel 

End of hole, 1 to 
2 ft of steel. .. 


117.5 

116 

115.5 


105 
101 
101.5 


12.5 
15 

14 



85 



muffler-enclosure accounted for about 11 
dBA of this reduction; the shroud tube 
and/or the rock mass surrounding the 
drill hole accounted for the remainder. 
Ice formation and damage to the noise 
control treatments were negligible. 

The retrofitted drill was then tested 
in an operating underground zinc mine. 
As shown in table 1 , noise levels at the 
operator's position were 12.5 to 15 dBA 
lower than with the untreated drill. One 
of the reasons for the more modest noise 
reductions in the underground tests was 
that the confined, reverberant under- 
ground environment set up reflections 
that partially negated the advantage of 
directing the exhaust air away from the 
operator. (Note also that the overall 
noise levels were much higher underground 
than aboveground . ) The noise levels of 
the treated drill show that it could have 
been operated for about 8 h per shift 
aboveground (88-92 dBA) and about 1-1/2 h 
underground (101-105 dBA) without violat- 
ing Federal noise regulations. 

The durability of the noise control 
treatments was evaluated by drilling ap- 
proximately 10,000 ft of hole in the un- 
derground zinc mine. Although only about 
500 ft were drilled with the shroud tube 4 
the muffler-enclosure was used during the 
entire test period. Overall, the compo- 
nents of the muffler-enclosure were quite 
durable; the outside was not damaged, the 
fiberglass baffles were in good condi- 
tion, the protective film had only two 
small holes, and the rubber exhaust de- 
flector showed no signs of wear. The 
only damaged acoustical component was the 
rubber seal at the drill air inlet , which 
came off when the bolts supporting the 
drill mounting bracket failed. This 
failure, however, was not the fault of 
the acoustical treatments themselves. 

Mine personnel reported very good oper- 
ator acceptance of the partially quieted 

4 The mine did not possess the 10-ft- 
long drill steels for which the shroud 
tube was designed; by the time the appro- 
priate steels were obtained, the test pe- 
riod was almost completed. 



drill (muffler-enclosure only) during un- 
derground tests, despite the need to fix 
damaged drill parts and support brackets 
on several occasions. The presence of 
the muffler-enclosure did not interfere 
significantly with either the replacement 
of broken parts or routine drill mainte- 
nance. Operators generally agreed that 
the treated machine drilled just as fast 
or faster than the unmodified drills used 
at the mine. 

The Bureau is presently testing the 
fully quieted drill at its Pittsburgh 
(PA) Research Center to evaluate the dur- 
ability of the shroud tube in figure 10 
and other similar shroud tube designs. 
The effect of the shroud tube on operator 
acceptance (e.g., the ability to observe 
a stoppage of drill steel rotation) is 
also being evaluated. 

Redesign of Jumbo Drill 

Although the retrofit muffler-enclosure 
described would be quite effective for 
most jumbo drills with rifle bar ro- 
tation, it would not be appropriate for 
drifters containing independent drill 
steel rotation motors. This is because 
independent rotation drills are usually 
somewhat larger than rifle bar drills, 
and would require larger, heavier 
muffler-enclosures. The problem of air 
exhaust from the rotation motor would 
also have to be addressed. Therefore, 
the Bureau sponsored a program to rede- 
sign an independent-rotation jumbo drill 
for the purpose of reducing noise. A 
prototype of the redesigned drill is 
shown in figure 11. 

In order to make a simple, compact 
muffler-enclosure for the drifter, the 
rotation motor was removed from the 
drifter body and relocated at the front 
end of the feed channel. This design 
change required the use of a specialized 
drill steel called a "kelly bar." The 
drifter supplied percussion to the rear 
end of the kelly bar while the new rota- 
tion mechanism (an air motor, belt drive, 
and gears) imparted rotation to its front 
end. A small muffler was placed on the 



86 




FIGURE 11. - Redesigned jumbo drill with collapsible shroud tube prior to drilling. 



exhaust hose of the rotation air motor to 
attenuate its noise. 

The modified drifter was then placed 
within a two-piece, boxlike enclosure 
made entirely of molded polymer material. 
The top half of the muf fler-enclosure fit 
snugly atop the bottom half , and could 
be removed for easy access to the drill. 
The drifter was mounted within the bot- 
tom half of the enclosure through rubber 
bushings that isolated the feed channel 
from drifter vibration. 

A shroud tube was also used on the re- 
designed jumbo drill to abate drill steel 
noise; however, its design was quite dif- 
ferent than that of the shroud tubes on 
the retrofitted jumbo drill and the rede- 
signed stoper. As shown in figures 11 
and 12, the shroud tube on the redesigned 
jumbo drill was a collapsible steel coil 
of ~8-in diam. Unlike the shroud tube on 
the retrofitted jumbo drill, it did not 
touch the drill steel nor enter the hole 
during drilling. Instead, it was sus- 
pended firmly between the front portion 
of the drifter enclosure and the rear 
face of the kelly-bar rotation mechanism. 



The springlike shroud tube was completely 
extended at the start of the drilling 
(fig. 11), and collapsed as the drifter 
moved toward the face (fig. 12). Ex- 
haust air from the drifter moved forward 
through the shroud tube and a plunger- 
shaped rubber "stinger" (fig. 13) that 
was pressed against the rock face. The 
drifter exhaust air, hole-flushing air, 
and rock chips produced during drilling 
exited through the small gap between the 
stinger and the rock face. The stinger 
helped attenuate noise that would have 
otherwise have "escaped" from the collar 
of the hole. 

Initial testing of the redesigned jumbo 
drill was conducted in a surface rock 
quarry (figs. 11-13) and in a nonproduc- 
tion setting at the Colorado School of 
Mines experimental mine. Noise levels 
at the operator's position were about 96 
dBA on the surface and about 100 dBA 
underground, a substantial reduction com- 
pared with 110 to 115 dBA of standard 
jumbo drills. The only significant prob- 
lems noted during these tests were 
(1) repeated failure of the air- and/or 
water-flushing tube (probably unrelated 



87 




FIGURE 12. - Redesigned jumbo drill with collapsible shroud tube at completion of drilling. 




FIGURE 13. - Plunger-shaped stinger and kelly-bar rotation system at front end of feed channel. 



88 



to the noise control treatments) and 
(2) inability to observe drill steel 
rotation. 

Before the redesigned jumbo drill is 
taken to an operating underground mine 
for further testing, it will be modified 
to facilitate longhole drilling, where 
numerous lengths of drill steel are used. 
With the present design, the shroud tube 
must be collapsed by hand in order to add 
drill steel, an awkward and somewhat dan- 
gerous process. The design modification 
will include an automatic drill steel 
changing apparatus that will improve both 
the productivity and safety of the rede- 
signed drill. Details of the drill steel 
changing mechanism and results of under- 
ground "production" tests will be docu- 
mented in future Bureau reports. 

CONCENTRIC DRILL STEELS 



be transmitted through different struc- 
tural members. 

3. Miners should readily accept con- 
centric steels because (a) the drill 
steel changing process will be no more 
complex than the present process and 
(b) they will be able to observe drill 
steel rotation at all times. 

4. Existing drills can be retrofitted 
easily to accept concentric steels. 

Construction of a prototype concentric 
drill steel is now underway, and field 
testing will begin in 1984. This first 
prototype has been designed to fit a pop- 
ular pneumatic drifter model; similar 
prototypes are now being designed for 
other pneumatic drifter models, a hydrau- 
lic drifter, and the "quiet" hand-held 
drills discussed earlier in this paper. 



Perhaps the most innovative technique 
for controlling drill steel noise is the 
concentric drill steel concept, now being 
investigated under Bureau contract (10) . 
The two basic components of the concen- 
tric steel are an inner "pulse transmis- 
sion rod" and an outer "torque tube." As 
their names imply, the inner rod trans- 
mits percussive energy to the bit but 
does not rotate, while the torque tube 
supplies rotation and acts as a shroud 
tube to attenuate noise produced by the 
inner rod. The torque tube is acousti- 
cally isolated from the inner rod by but- 
tonlike rubber inserts. The inner rod is 
solid, and hole-flushing air or water 
passes through the annulus between the 
rod and tube. 

The concentric drill steel has the fol- 
lowing distinct advantages over any other 
drill steel shrouding technique developed 
to date: 

1. Drill steel life should increase 
because the inner pulse transmission rod 
is solid (no blow tube) and will not be 
exposed to the high torques and external 
scratching that usually initiate failure. 

2. Less expensive steel alloys can be 
used because percussion and torque will 



IN-HOUSE BUREAU RESEARCH 
ON DRILLING NOISE 

During the past 2-yr, the Bureau's 
Pittsburgh (PA) Research Center (PRC) has 
acquired the equipment and facilities 
necessary to conduct extensive in-house 
research on percussion drill noise. Ini- 
tial tests are now being performed out- 
doors at PRC, using a concrete block as a 
drill medium, and underground at the Bu- 
reau's Lake Lynn Laboratory, an abandoned 
underground limestone mine. Detailed 
investigations of drilling noise will be 
conducted inside a reverberation build- 
ing that is now being constructed at PRC 
(completion scheduled for mid-1984). As 
in the past, Bureau research will focus 
on two basic areas — muffling of drill ex- 
haust noise and control of drill steel 
noise. 

Exhaust Mufflers 

In order to investigate the muffler 
freezing problem more closely, the Bureau 
has acquired (1) a three-boom jumbo rig, 
(2) two pneumatic drifters, (3) two of 
the "quiet" hand-held drills described 
earlier, (4) a portable air compressor, 
(5) a portable compressed air aftercooler 
and dryer, (6) a portable water injection 



89 



system, and (7) a complete, continuous 
airflow and water flow monitoring system. 
With the aid of these items , the Bureau 
will conduct controlled muffler freezing 
tests using a wide variety of drill and 
muffler designs. The noise-reducing cap- 
abilities and freezing characteristics of 
the various drill-muffler combinations 
will be documented and reported in subse- 
quent Bureau publications. 

Drill Steel Noise Controls 

The major in-house tool for investi- 
gating drill steel noise is a complete, 
portable hydraulic drilling system 



cently acquired by the Bureau. Because 
the hydraulic drifter produces no air 
exhaust noise, it is the ideal machine 
for this purpose. Initial investigations 
will focus on the durability, field- 
acceptability, and noise-reducing capa- 
bilities of "in-the-hole" shroud tubes 
similar to the tube on the retrofitted 
drifter described earlier. The front 
end cap of the hydraulic drifter has 
been modified to accept shroud tubes of 
various sizes and materials (plastics, 
metals, polymer materials, etc.). Con- 
centric drill steels and drill body en- 
closures for hydraulic drifters will also 
be evaluated using this machine. 



REFERENCES 



1. Visnapuu, A., and J. W. Jensen. 
Noise Reduction of a Pneumatic Rock 
Drill. BuMines RI 8082, 1975, 23 pp. 



6. Summers, C. R. , and J. N. Murphy. 
Noise Abatement of Pneumatic Rock Drill. 
BuMines RI 7998, 1974, 45 pp. 



2. Dutta, P. K. , and P. W. Runstad- 
ler. Development of Commercial Quiet 
Rock Drills. Ongoing BuMines contract 
J0177125; for inf. contact W. W. Aljoe, 
TPO, BuMines, Pittsburgh, PA. 



7. Creare Products, Inc. Develop- 
ment of Prototype Quiet Hard Rock Stop- 
er Drill. Ongoing BuMines contract 
HOI 13034; for inf. contact W. W. Aljoe, 
TPO, BuMines, Pittsburgh, PA. 



3. U.S. Mine Safety and Health Admini- 
stration. Noise Control Abstracts. Com- 
piled by MSHA Health and Safety Technol- 
ogy Centers, Denver, CO, and Pittsburgh, 
PA, 1983, 45 pp. 

4. Hawkes, I., and D. D. Wright. De- 
velopment of a Quiet Rock Drill. Volume 
1: Evaluation of Design Concepts (con- 
tract J0155099, Ivor Hawkes Associates). 
BuMines OFR 70-78, 1977, 95 pp.; NTIS PB 
283 774. 



8. Dixon, N. R. , and M. N. Rubin. 
Development of a Prototype Retrofit Noise 
Control Treatment for Jumbo Drills (con- 
tract HO387006, Bolt, Beranek and Newman 
Inc.). BuMines OFR 111-83, 1982, 106 
pp.; NTIS PB 83-218800. 

9. Creare Products, Inc. Develop- 
ment of Noise Control Technology for 
Jumbo Drills. Ongoing BuMines contract 
HO395025; for inf. contact W. W. Aljoe, 
TPO, BuMines, Pittsburgh, PA. 



5. Hawkes, I., D. D. Wright, and P. K. 
Dutta. Development of a Quiet Rock 
Drill. Volume 2: Sources of Drill Rod 
Noise (contract J0155099, Ivor Hawkes As- 
sociates). BuMines OFR 132-78, 1977, 77 
pp.; NTIS PB 289 716. 



10. Technological Enterprises, Inc. 
Development of Concentric Drill Steels 
for Noise Control of Percussion Drills. 
Ongoing BuMines contract JO338022; for 
inf. contact W. W. Aljoe, TPO, BuMines, 
Pittsburgh, PA. 



90 



CURRENT STATUS OF LOAD-HAUL-DUMP MACHINE NOISE CONTROL 
By Thomas G. Bobick 1 and Richard Madden 2 



ABSTRACT 



This Bureau of Mines paper reviews the 
initial noise control research conducted 
on a load-haul-dump (LHD) machine with a 
5-yd 3 bucket capacity. Additional noise 
control research has been conducted on 
five other vehicles — three 2-yd 3 - and 



two 8-yd 3 -capacity vehicles. This paper 
presents a description of the machines 
treated, a general discussion of the 
noise control treatments used, and a sum- 
mary of the acoustic and thermal perform- 
ance of the treatments. 



INTRODUCTION 



A 1975 Bureau study 3 identified load- 
haul-dump (LHD) machines as major con- 
tributors to the noise exposure of under- 
ground miners. Machines with no noise 
control treatments still produce sound 
levels on the order of 100 to 102 dBA. 
The high sound level coupled with long 
working times leads to a high noise expo- 
sure; machine operators are out of com- 
pliance with Federal noise regulations if 
they are exposed to these levels for more 
than 1.5 to 2 h. 

In an effort to reduce this exposure 
and bring these operators into compli- 
ance, the Bureau initiated a research and 
development program on noise control 



of LHD vehicles. The work has been con- 
ducted under three Bureau contracts : 
H0262013 by Bolt Beranek and Newman, 
Inc.; H0395076 by Eimco Mining Machinery 
International; and H0395041 by Lake 
Shore, Inc. Six machines, ranging in 
bucket size from 2 to 8 yd 3 , were quieted 
under these contracts. This paper pre- 
sents a description of the test machines, 
a general discussion of applicable noise 
control treatments , examples of the noise 
control treatments used, and a summary 
of the acoustic and thermal performance 
of the modifications. Information 4 is 
available that provides more detail on 
the treatments used on the test machines. 



MACHINE DESCRIPTION 



LHD vehicles are among the most wide- 
ly used machines in underground metal- 
nonmetal mining. These machines (figure 

^Mining engineer, Pittsburgh Research 
Center, Bureau of Mines, Pittsburgh, PA. 

2 Manager, Mechanical Systems Analysis 
Dept. , Bolt Beranek and Newman, Inc., 
Cambridge, MA. 

3 Patterson, W. N., 
A. G. Galaitisis. 
Powered Underground 
Impact, Prediction, and Control (contract 
H0346046, Bolt Beranek and Newman, Inc.). 
BuMines OFR 58-75, 1975, 227 pp.; NTIS PB 
243 896. 

4 Huggins, G. G., R. Madden, and B. S. 
Murray. Noise Control of an Under- 
ground Load-Haul-Dump Machine (contract 
H0262013, Bolt Beranek and Newman, Inc.). 



G. G. Huggins, and 
Noise of Diesel- 
Mining Equipment: 



1 shows an example) are low-profile 
diesel-powered loaders , with center ar- 
ticulation for short radius turns within 
the mine. Typically, the engine, torque 

BuMines OFR 125-78, 1977, 79 pp.; NTIS PB 
288 854. 

Daniel, J. H., J. A. Burks, R. C. Bar- 
tholomae, R. Madden, and E. E. Unger 
(comps.). Noise Control of Diesel- 
Powered Underground Mining Machines, 
1979. BuMines IC 8837, 1981, 29 pp. 

Walch, R. H., and G. L. Beech. Noise 
Control of Underground Load-Haul-Dump 
(LHD) Machines. Final report on BuMines 
contract H0395076 with Eimco Mining Ma- 
chinery International, 1984, 60 pp.; 
available from Thomas G. Bobick, Pitts- 
burgh Research Center, Bureau of Mines, 
Pittsburgh, PA. 



91 




FIGURE 1. - Typical load-haul-dump machine. 



converter, and transmission are on one 
side of the articulation pivot and the 
bucket is on the other side. Because the 
machine has the same tram capabilities in 
both forward and reverse, the operator 
sits on one side of the machine facing 
inward. Although the machines have many 
similarities, they do differ in bucket 
capacity, engine size, means of cooling 
the engine, and operator position. A 
comparison of the six machines considered 
in this program is presented in table 1. 

All six machines have planetary trans- 
missions that provide four speeds in both 
forward and reverse, and all but one are 
powered by air-cooled diesel engines. In 
four of the machines, the operator's seat 
is on the aft section, which contains the 
engine and transmission. Major differ- 
ences in the machines exist in bucket ca- 
pacity with corresponding differences in 
physical size and engine size. Although 
bucket and engine size differ by approxi- 
mately a factor of four, there is less 
difference in noise levels for the ma- 
chines prior to noise control treatment. 



^> 
o- 1 

QlaJ >_ 

o°-o 

tOE 

?% 

< CD 



105 



95 



85 



75 



65 



1 . 1 . 1 1 1 1 

ST-5A, 4th gear. 


ii 


x 1 " 


ii 


912 D, 3d gear-^ i\ J 


-1 


< 
m 


• x/^jUx/A V / 


; 


_r 


//*V-«. N-t. < 


— 


LJ 




i 


> 

L'J 


■ J\r Vl 


^ 


_1 

_l 
_l 
< 




- 


CC 


■ i jn 


— 


Ld 
> 

n 


- *<J 918, 3d gear— ~-~*\- 
■. 1 . 1 . 1 . 1 . 1 


, : 





80 200 500 1,250 3,150 8,000 

ONE -THIRD OCTAVE BAND CENTER 
FREQUENCY, Hz 

FIGURE 2. - A-weighted one-third-octave band 

sound pressure levels for three different LHD's. 

Figure 2 illustrates this using the 
results of tests on three unmodified 
vehicles. This figure presents the 
A-weighted one-third octave band noise 
levels measured at the operator's posi- 
tion for the 2-yd 3 -capacity Eimco 912D, 
the 5-yd 3 -capacity Wagner ST-5A, and the 
8-l/2-yd 3 -capacity Eimco 918. In all 
three tests , the machines were operated 



TABLE 1. - Charactersitics of LHD machines 



Wagner 
ST-2B 



Wagner 
ST-2D 



Eimco 
912D 



Wagner 
ST-5A 



Wagner 
ST-8A 



Eimco 
918 



Bucket size yd 3 . . 

Engine size hp . . 

Engine cooling method 

Transmission speeds 

Operator location, section 



2 

77 

Air 

4 

Bucket 



2 

77 

Air 

4 

Bucket 



2-1/4 

85 

Water 

4 

Engine 



5 
180 
Air 

4 
Engine 



8 

269 

Air 

4 

Engine 



8-1/2 

269 

Air 

4 

Engine 



92 



with their wheels raised so that they 
could be run in gear while remaining 
stationary. 

The shape of all three curves is simi- 
lar" up to 1,600 Hz. Differences below 
1,600 Hz result because the Eimco 912D 
and 918 were tested in a different labo- 
ratory (with different acoustic proper- 
ties) than the Wagner ST-5A. Differences 
above 1,600 Hz result from the fact that 
the 91 2D and 918 were operated in third 
gear, whereas the ST-5A was operated in 
fourth gear. The change in gear causes 
the spectrum for the 918 to peak at 1,600 
Hz whereas the ST-5A spectrum peaks one- 
third octave band higher. This transmis- 
sion noise peak does not occur in the 912 
data because 3/8-in rubber pads provided 



some vibration isolation for the trans- 
mission. The right-hand side of the fig- 
ure presents the overall levels for each 
machine. Note that the overall levels 
differ only 3 dBA despite the large dif- 
ference in machine sizes. 

Because the machines have the same ba- 
sic noise sources and the spectra are 
similar, the same general noise control 
treatments should be effective for all 
machines. This does not imply that the 
noise control treatments are interchange- 
able between machines, or that an effec- 
tive treatment for one machine will be 
equally effective for another. General- 
ly, however, the same type of noise con- 
trol treatments were installed on all 
machines . 



NOISE CONTROL 



GENERAL PRINCIPLES 

Importance of Attenuating 
Dominant Contributions 

In general, the noise from any one 
source reaches a person's ears via sev- 
eral paths — both direct airborne paths 
and reflections from various surfaces. 
In addition, sound may propagate along or 
through structures (in the form of vibra- 
tions). Just as repairing small holes in 
a leaking roof is useless unless the 
large holes are closed off, reducing the 
noise of lesser sources and paths has 
practically no effect on a person's expo- 
sure unless the contributions from the 
dominant sources and paths are first 
reduced. 

Major Sources and Paths 

In diesel-powered mining equipment, the 
engine generally constitutes a major 
source of noise. Engine noise may come 
from the exhaust, the intake, and the 
casing (that is, the block and acces- 
sories attached to it); also, the cooling 
fan may be a significant noise source. 
The transmission, drive train, and hy- 
draulic system also tend to be signifi- 
cant noise sources. 



As mentioned, noise radiated from the 
various sources may reach the operator by 
propagation through the air, either di- 
rectly or via reflections. In addition, 
vibrations produced by the engine and 
other mechanical components tend to trav- 
el through heavy structures (such as 
frame rails) to lighter structures, which 
then radiate sound somewhat in the manner 
of a loudspeaker. The relative impor- 
tance of the various noise sources and 
paths differs for different machine types 
and models. 

REDUCTION OF NOISE OF 
DIESEL-POWERED EQUIPMENT 

The noise exposure of a machine opera- 
tor may be reduced by obstructing the 
sound propagating between the primary 
noise sources and the worker. Practical 
and economical considerations generally 
do not permit one to modify the primary 
noise sources or to replace them with 
quieter ones (except relatively early in 
the development cycle of new machines). 
Consequently, practical reduction of the 
noise requires obstruction of the propa- 
gation paths as its first concern. In 
addition to blocking the airborne paths 
between the major noise sources and the 



93 



operator, the structural paths must be 
considered for completeness. 

Airborne Paths 

Full enclosures are typically the most 
efficient means of blocking the radia- 
tion of sound from sources such as en- 
gines or transmissions. The effective- 
ness of such an enclosure increases with 
the mass of its walls, and can be further 
increased if acoustically absorptive ma- 
terial is placed on the inner surfaces of 
the enclosure. 

Where cooling or access requirements 
do not permit the use of complete enclo- 
sures, partial enclosures or barriers may 
be used. These tend to be considerably 
less effective than full enclosures, 
because sound can propagate out of the 
openings (or around the edges) and be 
reflected back toward the operator. The 
effectiveness of a partial enclosure or 
barrier can also be enhanced by placing 
acoustically absorptive material on the 
surfaces that face the noise sources. 
Because the noise reduction obtained with 
a partial enclosure or barrier is usually 
not limited by the sound that passes 
through it, but by the sound that gets 
around it, increasing the barrier or 
enclosure mass (which reduces the sound 
passing through the structure) usually 
results in little additional noise 
reduction. 

Mufflers are devices that obstruct the 
propagation of sound out of pipes or 



ducts, largely by reflecting some of the 
sound back toward the source in such a 
way that the reflected pressure waves 
cancel the generated waves to a consider- 
able extent. An engine exhaust muffler 
must be matched to the engine so it is 
effective acoustically, yet does not pro- 
duce excessive backpressure. 

Like mufflers, silencers also obstruct 
the propagation of sound out of pipes or 
ducts (for example, from the air intake). 
Silencers, however, work by absorbing the 
sound internally, rather than by reflect- 
ing it. Thus, silencers generally con- 
sist of acoustically lined pipes or ducts 
with baffles or louvers of acoustically 
absorptive materials inside them. Si- 
lencers should be selected to provide the 
desired sound attenuation without exces- 
sive obstruction of airflow. 

Structureborne Paths 

The propagation of vibrations along 
structures, which may cause structural 
surfaces to radiate sound in loudspeaker- 
like fashion, usually can be obstructed 
efficiently by inserting vibration- 
isolation elements into the propagation 
path. Typically, such elements need to 
be much softer than the structures they 
join; they may consist of rubber mounts 
placed between a vibrating engine or 
transmission and its supports, or a flex- 
ible hose inserted in a run of rigid hy- 
draulic tubing. 



NOISE CONTROL TREATMENTS 



The noise control treatments are pre- 
sented in the following order: first, 
those applicable to the engine, its ex- 
haust, and cooling fan, then those appli- 
cable to the drive train (the transmis- 
sion and torque converter), and finally, 
general treatments installed at the oper- 
ator's position. The order of discussing 
the noise controls does not, however, 
define the order that these sources were 
treated. As stated previously, noise 
levels had to be measured so the sources 



could be rank-ordered according to inten- 
sity. Modifications were applied to the 
loudest sources first. A complete dis- 
cussion of the treatments for the Wagner 
ST-5A and its performance on the surface 
and underground is given in the first and 
second works cited in footnote 4. The 
third work cited in footnote 4 discusses 
the modification effort and the in-lab 
testing for the two Eimco LHD's; a sepa- 
rate volume 2 report will discuss the 
results of the in-mine testing. The 



94 



report or the modifications to the three 
other Wagner machines (ST-2B, ST-2D, ST- 
8A) is being finalized. 

ENGINE 

Enclosures 

The diesel engine compartment on each 
test vehicle was modified with an en- 
closure that was lined with absorptive 
material. Figure 3 shows the enclosure 



panels for the initial research program 
on the Wagner ST-5A. The enclosure con- 
sisted of a treated hood, side panels, 
and belly pan with ducts for cooling air 
exhaust. Figure 4 shows similar modifi- 
cations for one of the other vehicles 
treated during this research. 

Because enclosing an air-cooled engine 
creates a potential for over-heating, 
thermal tests were conducted. Results of 
this testing, discussed in the following 





H 






,\i 


\ ■PS?^ 11 ^ 


tjt, &J ~-*<w£ y| r*^^& 


:l 






i 


— " jS 


jMfcM 










FIGURE 3. - Components of engine enclosure for Wagner ST-5A. A, Hood; B, left-side 
barrier; C, right-side barrier; D, belly pan and cooling air exhaust slots. 



95 





FIGURE 4» - Left and right sides of engine compartment cover for Eimco 912D. 



section, indicate only modest increases 
in operating temperatures of the ST-2B 
vehicle after installation of the noise 
control treatments. 

Rubber gasketing material was applied 
to the edges of all the panels to vibra- 
tion isolate them from the frame of the 
LHD. The panels were lined with 1- or 2- 
in-thick acoustical absorption material 
and then protected by perforated steel or 
expanded metal. The absorptive material 
was attached to the panels by studs and 
cover buttons. The side panels of the 
ST-5A unit (figs. 3B and 3D) were de- 
signed with removable sections that were 
attached to the fixed sections with hex- 
head bolts. 

Exhaust 

Exhaust noise is predominantly low fre- 
quency noise that is best controlled by 
using a commercially available resonant 
muffler, usually supplied by the machine 
manufacturer or its distributor. If suf- 
ficient room is not available for a reso- 
nant muffler, passing the exhaust through 
a filled water scrubber has also been 
shown to be effective. 



Four of the five air-cooled machines 
were treated with mufflers. The other 
two vehicles successfully used the 
f illed-water-scrubber technique. Addi- 
tionally, five of the six machines had 
the exhaust manifolds wrapped with heat- 
resistant material to reduce the temper- 
atures inside the engine enclosures. 
Figure 5 shows the wrapped exhaust mani- 
fold for one of the two 8-yd 3 -capacity 
machines . 

Cooling Fan 

Acoustical modifications were installed 
at the fan area on three of the six ma- 
chines. Figure 6 shows the treatments 
used to quiet the cooling fan on the Wag- 
ner ST-5A. An acoustically treated baf- 
fle was attached to the fan grill. Addi- 
tionally, the areas adjacent to the fan 
had acoustical absorption material, which 
was covered by aluminized Mylar 5 polyes- 
ter film, attached to it by the stud- 
cover button method. Figure 7 shows a 

^Reference to specific products does 
not imply endorsement by the Bureau of 
Mines. 



96 



slightly different treatment for the 
Eimco 918. Similar to the ST-5A unit, a 
solid acoustically treated baffle was 
installed directly in front of the fan. 



Additionally, however, a series of acous- 
tically treated louvers were installed at 
the fan area to block the direct path of 
the airborne sound generated by the fan. 




FIGURE 5. - Modified exhaust system on one of the two 8-yd 3 -capacity LHD's. 





FIGURE 6. - Baffle for fan in lowered (left) and raised (right) positions. 



97 




FIGURE 7. - Engine cooling fan treatment for Eimco 918. 



The areas adjacent to the fan were also 
treated with absorptive material that was 
protected by expanded metal. 

DRIVE TRAIN 

Enclosures 

The cover to the transmission and 
torque converter compartments on each ve- 
hicle was treated with absorptive materi- 
al. Figure 8 shows the treatment for the 
Wagner ST-5A LHD; perforated metal pro- 
tected the Mylar polyester film facing of 
the acoustical material. Figure 9 shows 
the treatment of the Eimco 912D vehicle; 
the acoustical material utilized in this 
case had a steam-cleanable outer surface. 



The bottom of the ST-5A transmission 
compartment was sealed with an untreated 
steel belly pan. A drain hole was incor- 
porated so the transmission fluid could 
be emptied. Additionally, the open side 
of the transmission of the ST-5A, located 
at the machine's pivot point, was closed 
off with 10-gauge steel that had been 
treated with mass-loaded, acoustically 
absorptive material. 

Compartment Absorption 

The interior of the water and fuel 
tank, compartment on the Wagner ST-5A 
was treated with acoustical absorp- 
tion material. Approximately 14 ft 2 of 
the polyester-film-faced acoustical 



98 




FIGURE 8. - Transmission compartment cover for Wagner ST-5A. 



absorption material was stud -welded to 
the front, back, and side surfaces. Sim- 
ilarly, the interior of the torque con- 
verter compartment of the ST-5A was lined 
with acoustical absorption material. Ap- 
proximately 22 ft 2 of the film-covered 
material was attached to the front, back, 
and side surfaces with the stud-cover 
button technique (fig. 10). These com- 
partments were also treated on two of the 
three other Wagner vehicles. 

Additionally, the interior surfaces of 
the transmission compartment on four of 
the six test machines were treated with 
acoustically absorptive materials. 



Vibration Isolation 

As mentioned, vibration can propagate 
through the machine's structure and cause 
attached components to generate airborne 
noise. The required noise control tech- 
nique usually consists of interrupting 
the path of propagation. This can be 
accomplished by using vibration-isolation 
mounts or pads between the component gen- 
erating the vibrations and the structure 
through which the vibrations travel. 
Four of the six LHD vehicles had the 
mounting of the transmission changed 
from rigidly attached to the frame to 



99 




FIGURE 9. - Treated transmission hood of Eimco912D. 



being supported by resilient elastomeric 
mounts. Figure 11 shows the general ar- 
rangement of the mounts for the transmis- 
sion of the Eimco 918 and 912 vehicles. 

GENERAL TREATMENTS 

Sealing Operator Compartment 



All of the 
the operator' 
hide, the 
directly in 
the transmis 
units (the 
transmission 



LHD's had modifications to 
s compartment. For each ve- 
operator is located either 
front of or to the side of 
sion. For the three small 

ST-2B, ST-2D, 912D), the 
is immediately to the right 



of the operator; the larger vehicles (the 
ST-5A, 918, ST-8A) have the transmission 
located directly in front of the opera- 
tor. In each case, all openings that 
permitted noise to escape from the trans- 
mission compartment were sealed. Figure 
12 shows the untreated and treated ST-2B 
machine. Figure 13 shows the modifica- 
tion to the opening for the transmission 
fluid dipstick on the 912D vehicle. The 
dipstick was relocated to another area, 
which permitted this major opening to be 
sealed. A principal hole that required 
sealing on the ST-5A machine was around 
the steering column. Because this column 
had to move, the opening was closed with 



100 



a shroud that had an elongated opening 
that permitted the required movement. 

The two largest LHD's (the ST-8A and 
918) had more areas that could be treated 



than the three smallest vehicles. Figure 
14 shows the treatment of the metal sur- 
faces near the operator's legs for the 
Eimco 918. The acoustical absorption ma- 
terial was covered with expanded metal to 




FIGURE 10, - Absorptive material applied to the interior of ST-5A torque convertor compartment. 



Transmission mount 




Transmission 



Transmission 
mount 



Bolt assembly 



Fibromount 




Main 
frame 



Spacers 



Cinch 
washer 



Elastomer mount 



FIGURE 11, - Transmission isolation mounts for Eimco 918 (left) and 912D (right) machines. 






101 





FIGURE 12. - Operator's position, transmission untreated (left) and treated (right), Wagner 
ST-2B vehicle. 



- 




*yA 




, 




vj 


r 


■■ '■■■■■ 








F( 




m-~ 






|- 




1 m &ss 




- . ■ ,:■,,.. 








1 


i|| 


*mM 






■i 








FIGURE 13. - Operator's compartment, before (left) and after (right) modifications to the transmis- 
sion dipstick, Eimco 912D. 



102 




FIGURE 14. - Operator's compartment, acoustical material (covered with expanded 
metal) near the foot pedals of the Eimco 918 LHD. 



protect it. Additionally, the operator's 
compartment of the 918 was isolated from 
the machine frame by resilient, elasto- 
meric mounts, instead of the normal rigid 
mounting. Vibration measurements indi- 
cated a reduction in the noise level. 

Canopy Absorption 

Three of the six machines (the 91 2D, 
918, ST-8A) were equipped with an opera- 
tor's canopy for falling object protec- 
tion. A simple method for eliminating 
noise reflecting from the canopy back 
onto the operator is to treat the canopy 



with acoustically absorbing material, as 
shown in figure 15 for the Eimco 918 and 
912. The absorption material should be 
protected by perforated plate or with 
expanded metal. 

SUMMARY OF NOISE CONTROL TREATMENTS 

A summary of the noise control treat- 
ments applied to the six machines is pre- 
sented in table 2. The primary treatment 
applied to all machines was an engine en- 
closure with acoustical absorption mate- 
rial applied to the inside. The majority 
of machines also had some modification to 



103 



TABLE 2. - Summary of LHD noise control treatments and year of control installation 



Noise source and control treatment 


Wagner 

ST-2B, 

1981 


Wagner 

ST-2D, 

1981 


Eimco 
912D, 
1981 


Wagner 

ST-5A, 

1976 


Wagner 

ST-8A, 

1983 


Eimco 
918, 
1981 


Casing: 


X 
X 

X 

X 
X 
X 

X 
X 

X 
X 

X 
X 


X 
X 

X 
X 
X 

X 
X 

X 
X 
X 
X 

X 
X 


X 
X 
X 

X 
X 
X 

X 
X 

X 
X 
X 

X 

X 
X 


X 
X 

X 

X 
X 
X 
X 

X 

X 
X 
X 

X 
X 

X 


X 
X 

X 

X 
X 

X 

X 
X 
X 
X 

X 

X 

X 

X 
X 


X 


Absorptive treatment on enclosure.... 


X 






Cooling fan: 


X 




X 
X 




X 


Exhaust: 






x 




x 






Transmission and torque converter: 




Torque converter (or other) cover(s). 
Absorptive treatment: 


X 


Barrier between torque converter and 


X 




X 
X 


Operator compartment: 
Absorptive treatment: 


X 

X 
X 



the airflow entering the cooling fan 
either to improve performance by reducing 
recirculation or to eliminate a direct 
noise path from the fan. Exhaust muf- 
flers were installed on two of the three 
smallest machines. 

Typical treatments applied to the 
transmission included absorption material 
applied to all of the transmission com- 
partment covers and, where sufficient 
space existed, to the side walls of the 



compartment to eliminate airborne noise. 
Four of the six vehicles (the 91 2D, ST- 
5A, 918, ST-8A) had the transmission 
mounted on vibration isolators to elimi- 
nate struct ureborne noise. Vibration 
isolation is particularly important once 
the major airborne sources are quieted. 
For the operator's position, all vehicles 
had openings sealed off and, in cases 
where a canopy existed, sound absorbing 
material was installed to reduce sound 
reflections. 



104 





FIGURE 15. - Canopy treatment of the Eimco 918 (left) and 912D (right) machines. 



PERFORMANCE OF NOISE CONTROL TREATMENTS 



The performance of the noise conrol 
treatments is discussed in two sections. 
The first deals with changes in tempera- 
ture that resulted from the modifications 
and the second documents the acoustic 
performance of each of the fully treated 
vehicles. 

THERMAL PERFORMANCE 

Throughout the program, the Bureau 
tried to develop a simple laboratory 
thermal test that would predict the 
changes in temperature expected under ac- 
tual working conditions. Unfortunately, 
to date, none has been developed. There- 
fore, temperatures measured during actual 
operation were relied on to evaluate 
thermal performance. The output from a 
series of thermocouples was recorded us- 
ing a data logger secured to the test 
machine. The data logger was programmed 
to average the temperatures over a pe- 
riod of time, generally 10 min. Typical 
data are illustrated in figure 16 by a 
set of temperature measurements of engine 
oil, transmission oil, and ambient air, 



taken on the Wagner ST-2B. Particularly 
noteworthy is that approximately 100 min 
was needed for temperatures to stabilize. 
When conducting thermal tests , sufficient 
time must be allowed for twmperatures to 
stabilize. 

The key areas for temperature measure- 
ment are oil temperatures , air tempera- 
tures inside the enclosures, and ambient 
air. Oil temperatures are measured in- 
side the engine and at the inlet and out- 
let of the transmission oil cooler. Air 
temperatures are taken at a number of 
locations in the engine compartment and 
other compartments that are enclosed. 
Table 3 presents a comparison of the tem- 
peratures from the Wagner ST-2B before 
and after the installation of the noise 
reduction package. Data were taken in a 
working section that has a mean elevation 
of approximately 500 ft above sea level. 
These temperatures are averages of those 
measured during the stable portion of the 
work cycle; for example, temperatures be- 
fore treatment of the Wagner ST-2B are 
averages of data in the time range of 100 



105 



to 400 mln in figure 16. Temperatures 
have been normalized for an average ambi- 
ent temperature of 88° F. These data in- 
dicate an increase of less than 15° F in 
the oil temperatures and typically less 
than 35° F in the air temperatures in the 
enclosures. 

TABLE 3. - Critical temperatures on 
Wagner ST-2B before and after 
noise control treatment, degrees 
Fahrenheit 



Engine oil 

Transmission oil 
cooler: 

Inlet 

Outlet 

Compartments : 

Engine 1 

Battery 

Transmission. . . . 



Before 



214 



180 
158 

143 
141 
128 



'Average of 3 locations. 



After 



226 



193 
176 

175 
174 
166 



Change 



+ 12 



+ 13 
+ 18 

+ 32 
+ 33 
+ 38 



or 
!5 

IX. 

UJ 
0- 



250 



200 



150 



100 




*W^* 



50 - 



v ^ v ^>^wi 



KEY 
o Engine oil 
A Transmission oil 
17 Fan inlet air, ambient 



12 3 4 5 6 7 

TIME, 102 min 

FIGURE 16. - Typical temperature data for the 
Wagner ST-2B machine. 



ACOUSTIC PERFORMANCE 

A summary of the acoustic performance 
of the noise control modifications ap- 
plied to the six machines is presented 



in table 4, along with the various test 
conditions. The preferred measure of 
performance is actual working-shift noise 
dosimeter data. Table 4 presents dosime- 
ter data for three of the test vehicles. 



TABLE 4. - Effectiveness of noise control treatments, decibels (A-weighted) 





Wagner 


Wagner 


Eimco 


Wagner 


Wagner 


Eimco 




ST-2B 


ST-2D 


912D 


ST-5A 


ST-8A 


918 


SHOP 














High idle: ' 
















97 - 97.5 


NA 


101 


100-101 


NA 


99 




90 - 90.5 


NA 


91 


NA 


94 


90 


MINE 














High idle: 
















98 


98.5 


NM 


NM 


2 94.5-95 


3 NM 




93 - 95 


88 


91 - 92 


92.5 


90 -91 


94 


Operating: 
















4 101.5 


4 101-102.5 


NM 


NM 


NA 


NM 




4 96.5- 98.5 


4 95.5 


5 NA 


4 89- 91.5 


NA 


NA 



NA Not available. 

NM New machine treated; underground data not obtained prior to treatment. 

'Machine stationary, in neutral, maximum revolutions. 

2 Data obtained with some, but not all, treatments removed. 

3 After 1 yr all treatments removed; resulting noise level was 102 dBA. 

4 Corresponding average noise levels based on dosimeter data, L0SHA. 

5 Not yet placed into full-time production service. 



106 



Because three of the six vehicles were 
new units that were treated, no in-mine 
measurements were obtained for the un- 
treated condition of those machines. In- 
mine measurements, however, were obtained 

(maximum revolutions 
condition of all six 

The resulting noise 
level of the fully treated machines 
ranged from 88 to 95 dBA. Treated and 
untreated data were obtained for four 



for the high idle 
in neutral) test 
treated vehicles. 



vehicles; the noise reductions ranged 
from 4 to 10 dBA. An important point to 
keep in mind is that each 5 dBA reduction 
results in a doubling of the permitted 
operating time. Thus, reducing the oper- 
ator's full-shift noise exposure by 4 dBA 
results in a 75 pet increase in the per- 
mitted operating time, and a 10-dBA re- 
duction results in a quadrupling (300 pet 
increase) in the time a single operator 
may use the machine. 



CONCLUSIONS 



The Bureau's noise control program 
for load-haul-dump (LHD) machines has 
achieved a number of successes. The pri- 
mary noise sources have been defined and 
are generally the same for all machine 
types. Specific measurements have to be 
taken to determine the order of imple- 
menting the modification program. 

The larger units provide much more room 
than the smaller vehicles for installing 
the noise control treatments on existing 
equipment. The extra room facilitates 
lining the inner surfaces of the compart- 
ments with absorption material, and per- 
mits installation of vibration isolation 
mounts at the transmission. 



are properly maintained and are replaced 
or repaired when needed. 

Successful future noise control devel- 
opments will require close cooperation 
between the machine manufacturers and the 
mine operators. The manufacturers must 
be prepared to implement proven noise 
control treatments and modify designs 
that cause access or thermal problems. 
The mine operators , as the equipment 
users , can provide important feedback to 
the manufacturers on durability, inter- 
ference with maintenance requirements, 
and thermal performance of the noise con- 
trol treatments over the life of the 
machine . 



Most importantly, this research showed 
the importance of sealing all leaks, 
cracks, and holes in the operator com- 
partment. The acoustical effectiveness 
of well-designed, carefully installed 
treatments can be negated if all openings 
in the operator compartment are not thor- 
oughly sealed. Once noise-controlled 
LHD's are in use underground, it is im- 
portant that the acoustical modifications 



These research programs have shown that 
noise abatement of LHD machines has been 
successful. Regarding the two methods to 
incorporate noise control treatments — 
treating existing machines after they are 
in the mines or treating new vehicles as 
they are manufactured — the latter method 
is obviously the most efficient and cost 
effective over the life of the machine. 



107 



RETROFIT NOISE CONTROLS FOR CRUSHING AND SCREENING PLANTS 
By Terry L. Muldoon 1 and Thomas G. Bobick 2 

ABSTRACT 



Crushing and screening equipment in the 
sand and gravel and crushed stone indus- 
tries generate excessive noise. Plant 
operators and cleanup personnel receive 
a full-shift exposure that ranges from 
three to four times the exposure allowed 
by Federal regulations. Noise is typi- 
cally generated by the impact of the 
product against the steel components of 
the plant. The impact forces cause the 
components to resonate and create air- 
borne noise. 

During a Bureau of Mines sponsored re- 
search program, retrofit noise control 



treatments were successfully installed 
and evaluated in a primary crushing plant 
and two secondary crushing and screening 
plants. A control booth was installed at 
the primary crushing plant ; the noise 
levels at the operator's location were 
reduced from 97 to 78 dBA. Noise levels 
measured at normal cleanup locations were 
reduced by 4 to 7 dBA (97.5-98 to 91-93.5 
dBA) at one of the two secondary plants . 
This paper describes how to design, 
select, and install similar retrofit 
noise control treatments for crushing and 
screening plants. 



INTRODUCTION 



A 1981 Bureau of Mines study-* showed 
that the full-shift noise exposure of 
operators and cleanup personnel in 
crushing and screening plants were three 
to four times the exposure allowed by 
30 CFR, Part 56, "Safety and Health 
Standards — Sand, Gravel, and Crushed 
Stone Operations." The study also iden- 
tified the following major noise sources 
as the chief contributors to the overex- 
posure problem. 

a. Screen feed chute. Typically, ma- 
terial enters the screen through a steel 
chute from a belt conveyor. The product 
discharged from the conveyor impacts 
the sides, wall, and bottom of the steel 
chute. 

1 Manager, Mining Division, Engineering 
Systems Group, Foster-Miller, Inc., Walt- 
ham , MA . 

2 Mining engineer, Pittsburgh Research 
Center, Bureau of Mines, Pittsburgh, PA. 

3 Pokora, R. J., and T. L. Muldoon. 
Demonstration of Noise Control Techniques 
for the Crushing and Screening of Non- 
metallic Minerals (contract J01 00038, 
Foster-Miller, Inc.). BuMines OFR 50-83, 
1981, 187 pp.; NTIS PB 83-173039. 



b. Screen feedbox. Additionally, the 
product discharging from the screen feed 
chute impacts a steel screen feedbox that 
is an integral part of the screen. 

c. Screen. The normal screening medi- 
um is either punched steel plate or woven 
wire cloth. Some screens are furnished 
with steel side wings. High noise levels 
are generated by the impact of the prod- 
uct on both the deck and wing liners. 

d. Screen discharge. Typically, the 
oversize product from the top screen deck 
drops onto a steel discharge lip or di- 
rectly from the screen onto a steel plate 
in the crusher. The undersize product 
passes through the screening medium and 
impacts a discharge chute, transfer con- 
veyor, or another screen deck. 

e. Crusher feed hopper or chute. The 
feed to the crusher impacts a cylindrical 
or conical collection hopper that directs 
the feed into the crushing cavity. Often 
the feed to the crusher is sparse and the 
impacting product strikes the hopper in- 
dividually. A heavily fed (choke-fed) 
crusher has the opportunity for a bed of 



108 



material to build up and, 
tenuate the noise. 



therefore, at- 



f. Crusher feed plate. Most cone 
crushers are supplied with an abrasion- 
resistant metal feed plate. Product 
dropping into the crusher strikes the 
feed plate — particularly if the crusher 
is not choke fed. 



next comminution stage or to a stockpile. 
These sources are common to all crushing 
and screening plants and are accessible 
without major disassembly of the plant. 
The noise associated with each of these 
sources is generated by the impact of the 
product on the steel components of the 
plant, which then resonate, creating air- 
borne noise. 



g. Crusher feed cone. Typically, the 
feed cone is lined with manganese steel 
plate for wear. The product fed to the 
crusher strikes the feed cone. 

h. Crusher main frame. The shell sur- 
rounding the crushing cavity typically is 
impacted by product discharging from in- 
side the crusher. The shell acts as a 
radiator for all of the noise generated 
in the product reduction process from 
within the crusher itself. 

i. Crusher discharge. The product 
discharged from the crusher is typically 
transferred via another steel chute to a 
belt conveyor that transports it to the 



During the Bureau program, noise con- 
trol treatments were applied in two sec- 
ondary crushing and screening plants to — 

a. Minimize the impact forces. 

b. Enclose the source. 

At a primary crushing plant receiving 
run-of-mine product, a control booth was 
installed to enclose the plant operator. 

This paper describes how these treat- 
ments were applied, the costs associated 
with the treatments , and the noise reduc- 
tions achieved. 



PRIMARY CRUSHING PLANT 



NOISE CONTROL USING OPERATOR 
CONTROL BOOTH 

An extremely cost-effective noise con- 
trol treatment for stationary plant em- 
ployees is the construction of a control 
booth. A booth is not expensive to con- 
struct or purchase, can provide noise re- 
ductions of 15 to 25 dBA, requires little 
maintenance, and also helps protect the 
worker from the weather and other en- 
vironmental hazards such as dust. The 
construction or purchase of a booth is 
straightforward and quite a few have 
been installed by the industry. In many 
cases, however, they are not as effec- 
tive as they could be for the following 
reasons: 

a. The booth is not large enough. 

b. The booth does not have adequate 
air conditioning. 



c. The booth does not provide adequate 
field of view for the operator. 



d. The booth 
tight. 



is not acoustically 



e. The booth is not 
plant structure. 



isolated from the 



For a control booth to be effective, it 
not only has to reduce the noise, but al- 
so has to provide enough comfort so the 
operator will stay inside the booth dur- 
ing normal plant operation. An operator 
will not stay in a booth if it is too 
cramped, too hot, or does not provide 
adequate visibility. 

If a booth is to provide maximum noise 
reduction, it has to be acoustically 
tight. Even small leaks can reduce the 
noise reduction by 10 to 15 dBA. 



109 



Booth mounting is also critical. If 
the booth is mounted directly on the 
plant structure, the vibration from the 
plant, which is usually severe, will 
cause the booth structure to vibrate and 
radiate noise. If the booth has to be 
mounted on the structure, it should be 
mounted on correctly designed vibration 
isolators. A better alternative, if pos- 
sible, is to mount the booth on a sepa- 
rate structure that is not in contact 
with the plant. 

During the Bureau program, a booth was 
specified, purchased, and installed for 
the operator of a primary crushing plant. 
The plant uses a 16-ft by 42-in vibrat- 
ing feeder grizzly and a 32- by 42-in jaw 
crusher to process run-of-mine product 
from the quarry. The operator controlled 
this plant from an open catwalk over the 
crusher where noise levels averaged 97 
dBA. 




FIGURE 1. - Operator control booth mounted on 
separate steel support structure. 



NOISE REDUCTION AND COST 

The 8- by 10-ft (figs. 1-2) booth was 
purchased for $4,919. It was mounted on 
a separate structure that was constructed 
by quarry personnel using 6-in I-beams. 
The air-conditioned booth reduced the 
noise levels at the operator's location 
to 78 dBA, a 19-dBA reduction. A total 



of 40 h of quarry labor were required to 
fabricate and install the support struc- 
ture at the plant and to install controls 
inside the booth. Details of the physi- 
cal characteristics of the booth, and oc- 
tave band sound pressure levels measured 
inside and outside the booth are included 
in the final report of the work cited in 
footnote 3. 



SECONDARY CRUSHING AND SCREENING PLANTS 



NOISE CONTROL USING RESILIENT 
MATERIALS TO MINIMIZE IMPACT FORCES 



Specific treatments at the two plants 
included 



At the two secondary plants addressed 
during this program, resilient materials 
were used to minimize the noise produced 
by the product impacting the plant compo- 
nents. The two plants included 



a. Resilient impact pads for the wall 
and bottom of the screen feed chute. 

b. Resilient liner for the screen 
feedbox. 



a. A secondary plant that used a 5- by 
14-ft inclined double-deck screen, and a 
4-1/4-ft cone crusher. 

b. A secondary plant that used a 5- by 
14-ft horizontal double-deck screen, and 
a 5-ft cone crusher. 



c. Resilient screen decking. 

d. Resilient liners for the screen 
side wings. 

e. Resilient screen discharge lip. 



110 




FIGURE 2. - Primary crusher operator at his control station inside the booth. 



Ill 



f. Resilient liner for the crusher 
feed hopper. 

g. Resilient liner for the crusher 
feed cone. 

h. Resilient crusher feed plate. 

INSTALLATION OF IMPACT PADS ON 
WALL AND BOTTOM OF SCREEN FEED CHUTE 

Typically, screens receive product via 
a steel chute that is fed by a belt con- 
veyor. Product discharged from the con- 
veyor strikes the wall of the chute, re- 
bounds, and falls to the chute bottom 
where it discharges to the screen through 
the feedbox (fig. 3). Noise levels mea- 
sured near these chutes normally exceed 
110 dBA with a coarse product feed. 

The recommended noise control treat- 
ments for screen feed chutes include 

a. A resilient impact pad installed on 
the chute wall. 



installed, they not only reduce noise, 
but also significantly increase chute 
life. 

The impact pad for the chute wall can 
be either bolted to the wall or suspended 
in the chute. Figure 4 shows a profiled 
surface pad installation. Holes are 
drilled or burned through the chute wall 
and the pad is bolted in place. The pad 
should be the full width of the chute and 
should extend above and below the impact 
area. 



Proper impact pad selection 
the following information: 



requires 



a. The type and 
processed. 



size of product being 



b. The velocity of the product — for 
the chute wall, the belt speed should 
be adequate; for the chute bottom, the 
height of the drop is required. 

c. The dimensions of the chute. 



b. A resilient impact pad or a product 
dead bed used in the chute bottom. 

These treatments absorb the force of 
the impact; if properly designed and 



d. The angle of product impact. 

The last item, angle of impact is partic- 
ularly critical. The life of resilient 



9 



Screen 
feed - 
chute 
wall 



s°7^ 



b 



^•^^ Belt conveyor 
Dribble chute 




£3. 






Screen 
feedbox 
FIGURE 3. - Product feed path from the belt 
conveyor to the screen feedbox. 




FIGURE 4. - Installation of a profiled surface 
impact pad on the screen feed chute wall. 



112 



pads depends a great deal on the angle of 
product impact (fig. 5). At impact an- 
gles less than 70° the wear rate of a re- 
silient pad will be high. Above 70° the 
wear rate will be significantly better 
than steel. A profiled surface, shown in 
figure 4, can be used to increase the im- 
pact angle. 

The size, type, and speed of the prod- 
uct are used to determine the required 
pad thickness. Generally, the larger the 
product and the higher the velocity, the 
greater the thickness required to mini- 
mize crushing damage to the liner. 

An impact pad should also be bolted to 
the chute bottom as shown in figure 6. 
The boltholes in the pad should be coun- 
tersunk by the material manufacturer so 
the boltheads will be below the pad sur- 
face, as shown in figure 7. 

The chute bottom can also be protected 
by creating a dead bed, which is simply a 
buildup of product at the area of impact. 
A dead bed is also recommended in combi- 
nation with a resilient impact pad to 



Impact angle 




Resilient pad 

FIGURE 5. - Impact angle of product on 
resilient pad. 



improve the life of the pad and chute 
bottom. 

NOISE CONTROL TREATMENT OF SCREENS 

Typically, the product discharged from 
the feed chute impacts a steel feedbox 
that is an integral part of the screen. 
The product then passes over the screen- 
ing medium which is either punched steel 
plate or woven wire cloth. The product 
also impacts the steel side wings or the 
side-tension rails as it passes along the 
screen deck. At the discharge end of the 
screen, the product either passes over 
or falls onto a steel discharge lip, and 
then passes into the discharge chute. 




FIGURE 6. - Installation of a resilient impact 
pad on the bottom of the screen feed chute. 




mpact pad 

I 

//, Chute bottom 



FIGURE 7. - Fastening the impact pad to the 
bottom of feed chute with countersunk holes in 
the resilient material. 



113 



Noise levels measured beside screens han- 
dling coarse material often exceed 105 
dBA. 

The recommended noise control treat- 
ments for screens include 

a. Resilient linings for the screen 
feedbox. 

b. Resilient screen decking. 

c. Resilient liners on the side wings 
or side-tension rails. 



d. Resilient liners 
lip and chute. 



on the discharge 



Most screens are provided with a blank 
metal panel at the feed end preceded by a 
feedbox that is often protected by metal 
wear plates where the product impacts the 
screen. The blank panel should be re- 
placed by a thicker, blank resilient pan- 
el. A resilient impact pad should be 
installed in the screen feedbox to in- 
crease the thickness of the area that is 
impacted by the product discharge from 
the feed chute (fig. 8). 

The screening medium should be replaced 
by a resilient deck. When ordering re- 
silient decking, it is important to spec- 
ify the following information: 

a. Type and size of product being 
screened. 




FIGURE 8. - Resilient feedbox with impact pad 
where feed from the chute strikes the feedbox. 



b. The efficiency of the existing 
screen deck. 

c. The exact dimensions of the exist- 
ing deck. 

d. The type of mounting — whether the 
deck is bolted to the screen frame, or if 
it is held by side-tension rails. 

e. Type, location, and dimensions of 
screen support members. 

f. Type and dimensions of holddown 
clamping. 

In selecting a resilient deck, it must 
be remembered that the use of a resilient 
cloth may reduce screening efficiency and 
throughput. This can be caused by the 
resilient deck having less open area and 
being thicker than the metal deck. It is 
also critical to specify exact dimensions 
because resilient materials are extremely 
difficult to modify-to-fit in the field. 

If the deck is bolted to the screen 
frame (figs. 9-10), nonperf orated areas 
should be specified over the deck support 
members. The nonperf orations will pre- 
vent accumulation of product between the 
deck and support members , which can cause 
excessive wear of the frame. 

Resilient liners should also be bolted 
to the screen side wings and discharge 
lip, as shown in figure 11. The side 
wing liners should be at least 1 in thick 
and high enough to protect the side wings 
from product impact. The discharge lip 
liner should be the same thickness as the 
deck. The resilient liners for the sides 
of the discharge lip (fig. 12) should be 
thicker than those on the side wings. 
This will help funnel the screen dis- 
charge and prevent product from being 
jammed between the screen and the screen 
discharge hopper. 

The discharge from a horizontal screen, 
which feeds a crusher directly by choking 
the feed down to the opening size of the 
crusher feed hopper, requires a resilient 
liner on both the sides and bottom of the 



114 



I 

J. 



f© ©* do *© 




°o°o °o° 
o o o © 




^Q © o 

3o o°o°. 

3©°©®©° 
d © o o 

4 • 



© o © © 

'0°0 0°0 

[© o o 



4 « » 

©SO O^P 

a o © q 



o 

' - 





I 
l 
I 
I 



e 
«ssr 




FIGURE 9. * Installation of a bolted resilient 
screen deck. 




FIGURE 10. - Bolted resilient deck with blank 
impact panel in the feedbox. 



Side wing 



** 



Resilient tiner 



>■'*$ 




FIGURE 11. - Resilient liners bolted to the 
screen side wings and discharge lip. 

discharge chute (fig. 13). These liners 
are bolted in place with the boltheads 
countersunk in the resilient material. 

For screens installed using side- 
tension rails (fig. 14), the rails should 
also be equipped with resilient impact 
liners. The liners should be bolted or 
bonded to the rails. Trowel- or paint-on 
resilient coatings are not recommended 
because of their limited durability and 
effectiveness. 

In addition, screen support members 

require a resilient protective molding 

(bumper strip) to properly crown the deck 

and minimize wear (fig. 15). If a center 



115 




*^S^ 



FIGURE 12. - Resilient discharge lip with thick- 
er liners on the sides to funnel the screen discharge. 




FIGURE 13. - Resilient liner in a screen dis- 
charge chute directly feeding a crusher. 

clamping bar is used, a resilient molding 
for the bar should be used. Screen 
J-hook clamps should use a resilient 
block or ring to protect the nut and 
threads. 

The use of resilient screen decks can 
cause operating problems. As previously 
mentioned, screening efficiency may be 
decreased, which may require changes to 
the screen's throw amplitude, speed, and 
direction. In addition, the product 




FIGURE 14. - Resilient screen deck with re- 
siliently lined side-tension rails. 

tends to bounce more on a resilient deck, 
especially on an inclined screen receiv- 
ing coarse product. 

The higher bounce can create safety and 
feed distribution problems. To eliminate 
these problems, a drag curtain (fig. 16) 
should be installed over the feed chute 
discharge. The drag curtain should be 
made of heavy, abrasion-resistant, resil- 
ient material. It should be the full 
width of the screen and should extend to 
the end of the blank screen panel. Con- 
veyor belting is not recommended because 
it wears rapidly and does not have enough 
mass to retard the product flow. 

NOISE CONTROL TREATMENTS 
FOR A CONE CRUSHER 

A typical secondary crushing and 
screening plant uses a cone crusher to 
reduce oversized product from the screen. 
The oversized product is fed to the 
crusher feed cone from a steel hopper. 
High noise levels, typically over 110 
dBA, are generated when the product im- 
pacts the steel crusher components. 

The recommended noise control treat- 
ments include*- 



116 




Resilient rings 



Side-tension rail 



/ odd ocjo aDagaa qqq ' 

DDDDDQaaaaaaaga ' 
/ooDOOQGaacnaaQQa' 
/DDDDDDaaaaQaaaQ' 
/odd dod coo □qqqqq' 




Bumper strip 



FIGURE 15. » Installation of resilient screen 
deck using resilient side-tension rails, bumper 
strips, and blocks on the J-hook clamps. 



a. Resilient liners for a surge-type 
hopper, the feed cone hopper shell, and 
the feed cone. 

b. Resilient feed plate. 

c. Barrier curtain around the crusher 
main frame. 



Design of the liner for the crusher 
feed hopper requires the following 
information: 

a. Size and type of product. 

b. Drop height or velocity of the 
product at impact. 

c. Exact hopper dimensions. 

d. Angle of impact. 

The liner should cover all hopper sur- 
faces impacted by the product, not only 
during full-load operation, but also dur- 
ing screen startup and shutdown. It is 
also recommended that the liner (fig. 17) 
be suspended away from the hopper wall. 
This will reduce localized crushing 
forces on the liner and increase liner 
life. 







FIGURE 16. - Drag curtain installed on an in- 
clined screen. 







FIGURE 17. - Resilient liner installed in a 
crusher feed hopper. 



117 




FIGURE 18. - Installation of one-piece re- 
silient crusher feed cone liner. 




FIGURE 19. - Installation of resilient feed 
cone liner segments. 



Resilient liners are also recommended 
for the crusher feed shell and cone. 
Care has to be taken in sizing the liner 
thickness so the thickest liner possible 
can be used, and yet not cause interfer- 
ence with the material flow through the 
crusher. Ideally, the shell and cone 
liners should be a one-piece assembly 
(fig. 18). This one-piece assembly can 
be simply inserted over the existing 
shell and cone or replace the fabricated 
steel liner assembly. 

The lining can also be manufactured as 
segments for easier handling and attach- 
ing to the existing steel liners (fig. 
19). This is not recommended, however, 
because of the possibility of a segment 
coming loose and passing through the 
crusher, which can cause significant dam- 
age. Another benefit of the full assem- 
bly, particularly on crushers without a 
rotating bowl, is that the liner can be 
rotated for more even wear. 

For cone crushers with a steel feed 
plate, the plate should be replaced by 
one manufactured with resilient material 
(fig. 20). The new feed plate should be 
cast by the material manufacturer using a 
mold that matches the steel one. It is 
recommended, however, that the resilient 
plate be manufactured thicker and larger 
in diameter than the steel one to prevent 




FIGURE 20. - Installation of a resilient 
crusher feed plate. 



118 



premature failure of the material located 
between the holddown bolts and the out- 
side diameter of the plate. Additional- 
ly, the resilient feed plate should be 
manufactured with an integral steel cen- 
tering plate to match the machined female 
fit of the feed distributor or the main 
shaft nut. Replacing the steel plate 
with a resilient one will not affect the 
unbalanced forces of the crusher. 

To control the noise radiating from the 
crushing zone and from product impacting 
the main frame liner, an acoustical bar- 
rier curtain should be installed around 
the crusher exterior (fig. 21). The cur- 
tain should be fabricated from leaded 
vinyl that has a layer of absorptive ma- 
terial on one side. The material should 
be purchased wide enough to extend from 
the adjustment ring to the base of the 
main frame flange. Grommets should be 
specified along the top edge of the cur- 
tain to provide easy installation on 
bolts that can be welded to the adjust- 
ment ring. By attaching the curtain to 
the adjustment ring and letting it hang 
free, the curtain will allow vertical 



Fillet weld bolt to 
ajustment ring 



-» Cut out for countershaft, 
as needed 




Cut out for hydraulic 
lines, as needed 



FIGURE 21. - Installation of a barrier cur- 
tain around the crusher main frame. 



movement when the crusher passes tramp 
iron and will not interfere with normal 
crusher servicing. An installed curtain 
is shown in figure 22. Figure 23 pro- 
vides octave band spectra of the noise 
measured at one of the cleanup locations 
near the crusher treated with the barrier 
curtain. Additional details regarding 




FIGURE 22. - Noise barrier curtain installed 
around the crusher main frame. 



100 



GO 



UJ 
> 



UJ 

rr 

CO 
CO 

LU 

rr 

CL 



o 
co 



95 - 



KEY 
3/24, baseline 
3/26, crusher feed chute 
4/9, crusher feed cone liners 
7/21, curtain around crusher 




"250 500 1,000 2,000 4,000 dBA 

OCTAVE BAND CENTER FREQUENCY, Hz 

FIGURE 23. - Octave band analysis showing 
effect of the retrofit treatments, plant B, ground- 
level cleanup position, near crusher. 






119 



installing the noise control modifica- 
tions on the screen feed chute, screen 



decking, screen discharge, and the cone 
crusher are available from the Bureau. 4 



NOISE REDUCTION AND COST 



Noise measured in the near-field of the 
vibrating screen at both secondary plants 
showed a reduction of 5 and 8 dBA (113 to 
105 and 106 to 101 dBA) for the screen 
discharges and a reduction of 5 and 9 dBA 
(106 to 101 and 107 to 98 dBA) for the 
feed end of the screens. Crusher noise 
was reduced by 5 dBA (106 to 101 dBA) at 
one of the two secondary plants. 

The installation of the noise control 
treatments (resilient materials and bar- 
rier curtain) reduced the noise levels 
measured at the usual cleanup or mainte- 
nance locations by 4 to 7 dBA (97.5-98 to 
93.5-91 dBA) at one of the two second- 
ary plants. Cleanup-maintenance location 
noise levels were not significantly re- 
duced in the second plant because of the 
rapid wear of the installed materials. 



secondary crushing and screening plants 
treated during this program were $15,500 
(average) . Quarry labor required for in- 
stallation of the modifications averaged 
67.5 worker-hours. 

At one of the two secondary plants, the 
retrofit noise control modifications dis- 
played excellent wear characteristics. 
The rapid wear of the materials at the 
second plant was due to improper materi- 
als being supplied and changing circuit 
conditions. 

4 Pokora, R. J., T. G. Bobick, and T. L. 
Muldoon. Retrofit Noise Control Modifi- 
cations for Crushing and Screening Equip- 
ment in the Nonmetallic Mining Industry, 
An Applications Manual. BuMines IC 8975, 
1984, 24 pp. 



The costs, in 1981 dollars, for the 
treatments described for the two 



120 



NOISE CONTROL IN COAL PREPARATION PLANTS 
By Thomas G. Bobick 1 and Matthew N. Rubin 2 



ABSTRACT 



This paper presents the results of two 
recent Bureau of Mines sponsored programs 
related to noise control in coal prepara- 
tion plants. These programs were aimed 
at evaluating engineering controls for 
reducing the occupational noise exposure 
of plant workers. The first of these two 
programs evaluated the performance of 
various noise control techniques in an 
operating preparation plant. Four gen- 
eral categories of noise control treat- 
ments were selected, installed, and moni- 
tored over an extended period of time to 
evaluate their acoustic performance and 
durability. The four categories were re- 
silient screen decks, resilient impact 
pads, chute liners, and mass-loaded cur- 
tains. This program demonstrated that 



the treatments can be both effective 
(providing 5 to 10 dBA of noise reduc- 
tion) and durable (with effective service 
lives of 1 to 4 yr) . 

The second project focused on document- 
ing noise control treatments that can be 
incorporated into new coal preparation 
plants at the design stage. While some 
of the treatments considered were similar 
to those evaluated in the previous proj- 
ect, a number of additional techniques 
were also considered, such as equipment 
substitutions and changes in plant lay- 
out. Consideration of these techniques 
is possible for new plants because of the 
design flexibility provided during the 
planning stage of a new facility. 



INTRODUCTION 



The noise levels inside typical coal 
preparation plants often exceed 90 dBA, 
and occasionally exceed 100 dBA. 3 Be- 
cause preparation plant personnel can be 
exposed to these levels for appreciable 
portions of their work shifts, the noise 
exposures of these workers can often ex- 
ceed those permitted by current Fed- 
eral noise regulations.'* Recognizing the 
potential risk to the hearing of prepara- 
tion plant workers , the Bureau has spon- 
sored research into noise control tech- 
niques for coal preparation plants. 

1 Mining engineer, Pittsburgh Research 
Center, Bureau of Mines, Pittsburgh, PA. 

2 Senior engineer, Bolt Beranek and New- 
man Inc., Cambridge, MA. 

3 Ungar, E. E., G. E. Fax, W. N. Patter- 
son, and H. L. Fox. Coal Cleaning Plant 
Noise and Its Control (contract H01 33027, 
Bolt Beranek & Newman Inc.). BuMines OFR 
44-74, 1974, 99 pp.; NTIS PB 235 852/AS. 

^U.S. Congress. Federal Mine Safety 
and Health Amendments Act of 1977. Pub- 
lic Law 95-164, 91 Stat. 1317-1319. 



Much of the Bureau's early research has 
been directed toward noise control for 
existing plants , concentrating on identi- 
fying the major problems and evaluating 
retrofitable noise control treatments 
(such as resilient screen decks, impact 
pads , chute liners , and noise control 
curtains). This initial work produced a 
large amount of practical information on 
a variety of noise control treatments 
that can assist preparation plant opera- 
tors in reducing the noise levels in ex- 
isting plants. 5 

^Rubin, M. N. Demonstrating the Noise 
Control of a Coal Preparation Plant. 
Volume I. Initial Installation and 
Treatment Evaluation (contract H0155155, 
Bolt Beranak & Newman Inc.). BuMines OFR 
104-79, 1977, 182 pp.; NTIS PB 299 963. 

. Demonstrating the Noise Con- 
trol of a Coal Preparation Plant. Volume 
II: Long Term Treatment Evaluation (con- 
tract H0155155, Bolt Beranek & Newman 
Inc.). BuMines OFR 143-83, 1982, 91 pp.; 
NTIS PB 83-237354. 



121 



More recently, the Bureau has investi- 
gated noise control techniques for new 
coal preparation plants. 6 New plants of- 
ten require a different noise control 



approach than do existing plants, because 
of advances in plant design; also more 
flexibility exists for equipment selec- 
tion and layout during the design stage. 



NOISE CONTROL FOR EXISTING FACILITIES 



This project (contract H0155155) ex- 
amined the benefits and limitations of a 
variety of noise control treatments and 
materials through in-plant tests. Al- 
though such tests do not permit the same 
degree of control and documentation as 
laboratory tests, it was felt that data 
obtained from actual use in commercially 
operating preparation plants would be 
more realistic, and thus more useful to 
the industry. 

PLANT DESCRIPTION 

The plant selected for this demonstra- 
tion project was the Consolidation Coal 
Co. Georgetown preparation plant. 

The Georgetown preparation plant was 
built in 1951 and was designed to pro- 
cess 1,650 tons of raw coal per hour. 
Although the plant was originally de- 
signed to clean both surface and under- 
ground coal, there was a distinct shift 
toward surface-mined coal during the 
course of this project. 

The plant was designed with three basic 
cleaning circuits: 1-1/2 by 7 in, 3/8 
by 1-1/2 in, and by 3/8 in. As shown 
in figure 1, the raw coal entering the 
plant is first fed to a primary shaker 
screen where the oversized material is 
scalped off and crushed. The secondary 
sizing screens then separate the flow 
into the three size classifications. The 
large material from the top deck of 
the secondary screens is cleaned in two 

6 Rubin, M. N., A. R. Thompson, R. K. 
Cleworth, and R. F. Olson. Noise Control 
Techniques for the Design of Coal Prepar- 
ation Plants (contract JO100018, Roberts 
& Schaefer Co. and Bolt Beranek & Newman 
Inc.). BuMines OFR 42-84, 1982, 135 pp.; 
NTIS PB 84-166180. 



McNally-Baum 7 jigs, and then sized and/or 
crushed before loadout. The middle size 
cut from the secondary screens is cleaned 
in two Chance sand flotation cones. The 
clean coal is then dewatered and sized 
on two clean coal desanding shakers and 
either sent to Wemco centrifuges for dry- 
ing (for the smaller material) or loaded 
out directly (for the larger material) . 
The fine coal from the secondary screens 
is cleaned on Deister tables and dried in 
Reineveld centrifuges before loadout. 

NOISE CONTROL STRATEGY 

The basic noise control strategy was to 
balance the need for operational data on 
a variety of commercially available mate- 
rials with the desire to provide a mea- 
sure of noise reduction in the demon- 
stration plant. After the demonstration 
plant was selected, a noise and opera- 
tional survey was conducted to (1) iden- 
tify the major noise sources within the 
plant, (2) determine the noise exposures 
of plant personnel, and (3) obtain opera- 
tional data on maintenance, access, and 
visual-monitoring requirements. Because 
the selection of equipment to be treated 
was based on worker exposure, as well as 
the need for performance data on a vari- 
ety of commercially available noise con- 
trol materials , plant areas were categor- 
ized as either type I (continuous), type 
II (partial) , or type III (limited) ac- 
cording to the exposure time of plant 
personnel. 

Those pieces of equipment located in 
type' I or II areas and having high sound 
levels were considered high priority 
sources in the selection of equipment for 

'Reference to specific brand names does 
not imply endorsement by the Bureau of 
Mines. 



122 



-From 1,500-ton raw coal bin 



Plus 7-in mesh 



McNally 




1 1 1 1 1 



Blending and mixing M*BWaa|riMHHBiB 

conveyor 



Flash 

dryers ^^^> 

® 



FIGURE 1. - Flow chart of Georgetown preparation plant. 




Loading trocks 
and boding booms 



treatment. In general, this program con- 
centrated on treatments for screens, 
chutes , and dryers . 

NOISE CONTROL TREATMENTS 

The majority of the noise control 
treatments selected for use in the demon- 
stration plant fall into the following 
four categories: 

1. Resilient screen decks. 

2. Resilient impact pads. 

3. Chute liners. 

4. Mass-loaded vinyl curtains. 

Vibrating screens are probably the 
largest and most difficult to control 
noise source in coal preparation plants 



in general, and in this demonstration 
plant in particular. For the older, low- 
speed , crank-arm shakers , the primary 
noise generating mechanism is the impact 
of the material flow on the metal screen 
deck. In modern, high-speed, eccentric- 
weight screens, the noise generated by 
the drive mechanism can also be a major 
contributor. 

A number of manufacturers produce 
screen decks with a resilient (elasto- 
meric) top surface that is intended to 
reduce the impact noise generated by the 
material flowing over the deck. To eval- 
uate this feature, a variety of resilient 
screen decks were selected for testing. 
Because a redesign of the screen's drive 
mechanism was beyond the scope of this 
retrofit project, resilient screen deck- 
ing was the primary screen modification 
investigated. 



123 



Although the initial tests in the 
Georgetown plant verified that these re- 
silent decks were capable of reducing the 
coal-screen impact noise (fig. 2), sev- 
eral operational problems were also iden- 
tified. These were blinding (particular- 
ly for the thicker decks on the crank-arm 
shakers), and delamination of the resili- 
ent top surface of the elastomer-clad 
steel decks. To determine if these oper- 
ational problems were common to other 
plants, and if the newer resilient decks 
(which had come on the market during 
the monitoring period) had improved over 
those initially tested, supplementary 
screen deck tests were conducted in four 
other preparation plants. 

These supplementary screen deck tests 
were performed in conjunction with Hen- 
drick Mfg. Co. and Laubenstein Mfg. Co., 
two of the major screen manufacturers 
in the United States. Each company made 
arrangements for testing with two coal 
preparation plant operators , provided the 
screen decks to be tested, and arranged 
for one of its representatives to super- 
vise the installation and monitor the 
performance of the test decks. 

Each screen manufacturer provided rep- 
resentative samples of the two most 
common types of elastomer-clad decks 
produced at the time. For Hendrick, 
these included: (a) a 48-durometer Gates 
SBR rubber that was vulcanized to steel 
punch plate, and (b) a 40-durometer Lina- 
tex natural rubber cold-bonded to the 
steel base plate. Laubenstein' s decks 
were manufactured from an 80-durometer 
Tuffgard polyurethane that was cast onto 
a steel punch plate, and a 40-durometer 



9 . 110 

2 _1 CM 

2 UJ E 

° > V. 



CO 



LlI 



a 3* 100 
lu co w 90 - 

> CO » 

P or m 
on. -o 80 



I ft above steel 
screen deck 




ft above resilient, 
screen deck 



J < 



31.5 125 500 2,000 8,000 

OCTAVE BAND CENTER FREQUENCY, Hz 

FIGURE 2. - Sound pressure levels measured 
over steel and resilient screen decks. 



Linatex natural rubber cold-bonded 
steel punch plate. 



to a 



In these supplementary screen deck 
tests, blinding was not found to be a 
significant factor in any of the four 
plants in which the tests were performed, 
and delamination was evident in only one 
of the four preparation plants. Although 
the service life of the screen decks 
varied significantly from one plant 
to the next, the urethane-cast-to-steel 
decks proved to be particularly durable, 
providing almost 2 yr of service in 
one plant and 1.5 yr in another. In 
fact, in the latter plant, the panels 
screened more than 1.5 million tons of 
coal, and lasted approximately five times 
longer than the original steel decks 
(fig. 3). The panels were eventually 
removed because of cracks in the steel 
backing rather than wear of the urethane 
coating. 

Resilient impact pads were installed in 
the Georgetown demonstration plant at the 
discharge of various belts, basket eleva- 
tors, screens, and chutes to reduce the 
noise generated when the material flow 
impacted the steel chute walls (fig. 4). 
The impact pads selected were primarily 
rubber or polyurethane compounds. Both 
flat and profiled (i.e., ribbed) configu- 
rations were used, depending upon the 
impact angle. 



o 

in 

O 



LU 
CO 



UJ 


UJ 


> 


rf 


UJ 




_l 


$ 


Q 


3 


LlI 


Li. 


1— 

I 


UJ 
> 




(- 


LU 


< 


^ 


_l 




3 



-i 1 1 1 r 

Urethane feed panel 
replaced with one 
with larger holes 



— I — ' — i — i — i — <- 
Urethane discharge 
panels from screen A 
installed on feed end 
of screen B 
Worn area on 
rubber feed panel, 
hole elongation 




"cp- 



All urethaneJ 

panels removed 

because of cracks 

in steel backing 



Slight hole 

elongation on 

new rubber 

feed panel 



6 8 10 12 
TIME, months of service 



14 



16 18 



FIGURE 3. - Service history of test decks in plant D. 



124 



Rubber 

impact 

pad 




FIGURE 4. - Impact pad and chute liner installations. 



greater noise reduction potential than 
the rigid materials. Figure 5 illus- 
trates the noise reduction achieved with 
rubber chute liners in a closed chute. 
All of these materials, however, had only 
a limited effectiveness in open chutes 
because of the noise inherent in the 
material flow. The plastic tiles were 
found to be quite durable when subjected 
to smooth, sliding flows, but wore quick- 
ly when exposed to tumbling or impacting 
flows. The ceramic tiles, while more 
durable in tumbling flows, did show evi- 
dence of cracking over time. The rubber- 
lined chutes that handled 1-1/2- by 
3/8-in-material also proved to be quite 
durable as long as the rubber was care- 
fully bonded to the chute walls. 



Experience at this demonstration plant 
indicated that these pads were not only 
effective in reducing the noise resulting 
from the impact of the material flow, but 
they could also be a cost-effective solu- 
tion. That is, when designed and in- 
stalled properly, the service life of 
these impact pads can sufficiently exceed 
that of the original steel plates, which 
would compensate for their higher initial 
cost. These tests also confirmed that 
impact angle and pad thickness are the 
primary design parameters that must be 
carefully chosen to achieve maximum per- 
formance from the pads. 

Because the noise generated by the 
continual impact of material flow on 
steel chute walls is a major noise prob- 
lem in many plants , including the demon- 
stration plant, several types of chute 
linings were selected for evaluation. 
Information was also sought on the ser- 
vice life of these materials since some 
are sold on the basis of extended service 
life (as compared with ordinary steel 
life) , in addition to their noise reduc- 
tion potential. 

The chute lining materials evaluated 
included ultrahigh molecular weight plas- 
tic and ceramic tiles, as well as sheet 
rubber. The materials were installed 
in both open and closed chutes. As 
expected, the resilient materials had a 



Finally, these tests also confirmed 
that simply installing covers on open-top 
chutes can be a very effective, yet rela- 
tively low cost, noise control treatment. 
It should be recognized, however, that 
this treatment can make visual monitor- 
ing more difficult and therefore must be 
carefully evaluated on a case-by-case 
basis. 

Flexible curtains installed on overhead 
tracks were used to enclose or sepa- 
rate noisy equipment that could not be 
treated effectively through other means. 
These curtains have a number of advan- 
tages over rigid enclosures (such as 
adaptability to dense, complicated equip- 
ment layouts and ease of opening or re- 
moval for access and maintenance) which 
are particularly desirable in coal 
preparation plants. Of concern in this 
evaluation was not only the noise reduc- 
tion potential, but how durable they were 




m*9 

PKdd 80 

OO.T3 

o 



_L 



KEY 
Unlined steel chute, 103 dBA 
Rubber lined chute, 98 dBA 

I I L 



31.5 125 500 2,000 

OCTAVE BAND CENTER FREQUENCY, Hz 



8,000 



FIGURE 5. - Sound pressure levels measured 
6 in. from unlined and lined discharge chutes. 



125 



and whether their use imposed any sig- 
nificant operating restrictions on the 
plant. 

The curtains used in this project (pri- 
marily fiberglass reinforced, 3/4-lb/ft 2 , 
mass-loaded vinyl) proved to be both ef- 
fective from a noise control point of 
view, and very durable. The Velcro hook- 
and-loop closures were also found to be 
quite durable when sewn, rather than 
glued, on the curtains. Figure 6 illus- 
trates the noise reduction achieved by a 
typical installation. While the presence 
of the curtains did require that opera- 
tors open them to make visual inspec- 
tions, this was far easier than with rig- 
id enclosures. The curtains did not have 
a major impact on plant operation. 



id x 

o CM 

00 
■o 



< 

CD 

LU 

CJ 

o 



LlI 
> 

LlI 



60 



-i 1 1 1 1 — 

Inside closed curtains 

(985dBA) Curtains open 
(95.5 dBA) 




Outside closed curtains 
(89.5 dBA) 



31.5 125 500 2,000 8,000 

OCTAVE BAND CENTER FREQUENCY, Hz 

FIGURE 6. - Sound pressure levels measured 
at Wemco dryer curtains. 



NOISE CONTROL FOR NEW FACILITIES 



As indicated previously, new prepara- 
tion plants often require a different 
noise control approach than do existing 
facilities. This stems from advances in 
plant design, as well as the ability to 
make changes in equipment selection and 
layout during the design stage. While 
advances in coal preparation technology 
occur relatively slowly, the mix and type 
of equipment being used in new coal prep- 
aration plants is constantly evolving. 
Furthermore, it is sometimes possible, 
during the design stage of a new plant, 
to locate equipment and modify specifica- 
tions to compensate for the effect of 
noise control treatments. Although there 
are practical limits to such changes , 
this design flexibility can shift the 
balance when evaluating noise control al- 
ternatives. In existing plants, the cost 
of such modifications can be prohibitive 
and, therefore, limit the noise control 
options available. 

Considering the differences between 
noise control approaches for new and ex- 
isting preparation plants, the Bureau 
initiated a second project (contract 
J0100018) to study and document those 
noise control techniques that are suit- 
able for new preparation plants. 



Worker noise exposure can be minimized 
in new preparation plants by both care- 
ful plant layout and design, and treat- 
ment of individual equipment. Effective 
techniques during plant layout include 
isolation of high-noise areas, modifica- 
tion of personnel traffic patterns, and 
possibly even the selection of alterna- 
tive processes. Equipment treatment can 
include selection of low-noise models as 
well as retrofit treatment of standard 
equipment . 

PLANT LAYOUT AND DESIGN 

Isolation of noisy equipment can be im- 
portant for both mobile and stationary 
plant personnel. Ordinarily, the noise 
produced by equipment such as screens , 
chutes , crushers , and centrifuges propa- 
gates from floor to floor through open 
gratings , machinery wells , and stairways , 
thus keeping most of the plant at or 
above 90 dBA. The result is that super- 
visors , mechanics , and other mobile per- 
sonnel accumulate unnecessary noise dos- 
ages as they move about the plant. In 
addition, it is not uncommon to find shop 
areas located adjacent to high-noise 
equipment , such as vacuum pumps . In such 
cases , shop personnel may accumulate 



126 



noise dosages even though their own work 
in the shop may be relatively quiet. 

Providing the necessary isolation at 
the design stage through careful plant 
layout, partitioning off machinery wells 
and stairways, and floor-to-floor isola- 
tion (e.g. , through the use of concrete 
floors) is generally more cost effective 
than resorting to retrofit treatments af- 
ter the plant is operating to achieve the 
necessary noise reduction. Figure 7 il- 
lustrates one relatively simple method of 
isolating a main stairway and machinery 
well from a noisy screening floor. 

For equipment that will need to be en- 
closed, either individually or in groups, 
careful positioning along exterior walls 
(or preferably in a corner of the build- 
ing) can minimize the wall construction 
costs, as well as any interference with 



worker traffic patterns, lighting, pipe 
runs, etc. Positioning enclosed pieces 
of equipment along outside walls also 
simplifies ventilation design for the 
equipment and facilitates exterior vent- 
ing for blowers. Attended equipment and 
control panels should also be carefully 
located to minimize unnecessary noise ex- 
posure of personnel. For example, the 
relatively quiet flocculent mixing sta- 
tion located on the right side of figure 
7 , which must be attended several hours 
per day, would be better located on a 
quieter floor, or placed in a corner to 
reduce the cost of isolating it from the 
screening noise. 

Remote monitoring of noisy equipment 
can also reduce unnecessary noise expo- 
sure of plant personnel. While video 
cameras have occasionally been used in 
some operations, and computer-controlled 



Elevator 



Machinery 
well 



Heavy media 
vessel 



rOverhead door 



Drain 
and rinse 




^Secondary 



magnetic 
separator 



Qjg— — Flocculent 
mix tank 



Secondary 
magnetic 
separator 



Heavy media 
vessel 

FIGURE 7. - Sample floor plan of screening floor. 



127 



equipment monitoring is becoming more 
common, these systems can be relatively 
expensive. They are, however, not the 
only possibilities. One simple technique 
is to use enclosed, gallery-type observa- 
tion walkways to provide plant personnel 
with the desired visual access without 
exposing them to excessive equipment 
noise. Figure 8 shows, conceptually, 
what one of these galleries might look 
like. Ideally, the walkway would be 
linked to isolated stairways as described 
previously. 

Although noise is not normally a 
primary determinant in process design 
and equipment selection, some of these 



decisions will have a direct effect on 
the noise control requirements for the 
plant. For example, while the noise lev- 
el of process equipment often increases 
with the size, it is sometimes easier to 
isolate a few large pieces of equipment 
rather than many smaller units. The use 
of multiple pieces of equipment also 
tends to distribute the noise throughout 
the plant, which can make the design 
of some noise control treatments more 
complicated. 

In terms of how equipment selection 
can affect the plant noise control re- 
quirements , a good example is fines dewa- 
tering equipment because the noise varies 




FIGURE 8. - Schematic of gallery-type walkway. 



128 



significantly between specific types. Of 
the commonly used in-plant equipment , 
vacuum pumps for disk filters tend to be 
the noisiest, and are capable of produc- 
ing noise levels over 100 dBA in pump 
rooms; belt and filter presses can be 
some of the quietest equipment, capable 
of operating at less than 90 dBA at typi- 
cal operator positions. 

NOISE CONTROL OF INDIVIDUAL EQUIPMENT 

Although careful layout and design of a 
new plant can simplify and/or reduce the 
noise control requirements for individual 
equipment, generally it will still be 
necessary to specifically reduce the 
noise of some plant equipment. There are 
two basic alternatives for noise reduc- 
tion of plant equipment; (1) specify and 
purchase low-noise models , where avail- 
able, and (2) treat the standard equip- 
ment using retrofit treatments. There 
will also be instances when a combination 
of the two represents the most appropri- 
ate approach. 

Electric motors are probably the best 
example of equipment for which low-noise 
models are commercially available. While 
not the noisiest equipment in preparation 
plants, electric motors are used through- 
out the plant and can result in a sig- 
nificant noise problem, particularly when 
grouped together (such as on a pump 
floor) . 

The design features that are incorpor- 
ated into low-noise motors include 
low-noise fans (typically unidirectional 
fans), class F insulation that allows the 
motor to operate with less ventilation, 
better aerodynamic design of the housing 
and end frames , and better bearings for 
reduced mechanical noise. These features 
result in motors that are significantly 
quieter than standard designs; this is 
evidenced by comparing the sound spectra 
shown in figure 9 for comparable Westing- 
house 150-hp TEFC motors. 

Furthermore, some of the features that 
make a motor quiet also make it more 
efficient, and some manufacturers have 
incorporated these low-noise features 



into their line of energy-efficient mo- 
tors. For instance, GE's line of energy- 
efficient motors are approximately 3 pet 
higher in efficiency than its standard 
motors and produce sound levels compar- 
able to its low-noise motors. These mo- 
tors are about 25 pet more expensive than 
the standard units; depending upon hours 
of operation and power cost, they can 
have payback periods on the order of 1 to 
4 yr. The noise control, therefore, can 
be obtained at no extra cost in the long 
run. 

Vacuum pumps are another example of 
equipment for which low-noise versions 
are commercially available. Currently, 
it is possible to purchase treated ver- 



o 

(O-l ° 

LlI -°, 

Q > =1 



100 



co 



O cc 

Q. 




Ill 
> 

Eu 

_i 

a 

x 
* - 

UJ 



125 500 2,000 8,000 

OCTAVE BAND CENTER FREQUENCY, Hz 



FIGURE 9. - Comparison of octave band sound 
pressure levels for a TEFC Westinghouse 1,800- 
rpm, 150-hp standard motor versus quiet-line mo- 
tor (typical no-load sound pressure levels at 3 ft 
in a free field). 



o 

(Oj o 

< LlI CM 
CD- 1 O 

LULU 
H CO 

ow 

LU 

cr 

Q. 




50 



KEY 

Medium sized, 76 dBA 
Large sized, 75 dBA 



31.5 125 500 2,000 8,000 

OCTAVE BAND CENTER FREQUENCY, Hz 

FIGURE 10. - Sound spectra for Siemens ELM0- 
F vacuum pump. (Adapted from Siemens AG data 
sheet E-726/1086-101.) 



129 



sions of the commonly used positive dis- 
placement, rotary-lobe pumps, as well as 
liquid-ring vacuum pumps that are inher- 
ently quieter than the positive dis- 
placement pumps because of basic design 
differences. Figure 10 shows the sound 
spectra that Siemens specifies for two of 
its typical liquid-ring pumps. Liquid- 
ring vacuum pumps, however, are more ex- 
pensive than positive displacement types , 
and require seal water with relatively 
low solids content and neutral pH. 

As discussed earlier, retrofit noise 
control treatments for preparation plant 



equipment have been the subject of Bureau 
investigations for some time. Work cited 
in footnotes 5 and 6 provide detailed 
discussions of a number of these treat- 
ments. Not only are these treatments ap- 
plicable to new preparation plants, many 
are also easier to incorporate into new 
plants because of the flexibility avail- 
able at the design stage. Resilient 
chute linings and impact pads are notable 
examples since the sizes, angles, and 
mounting configurations of the chutes can 
be optimized at the design stage to fa- 
cilitate the use of these materials. 



SUMMARY 



This paper, of course, discusses only a 
few of the wide variety of noise con- 
trol techniques available to preparation 
plant operators and designers. Many of 
the retrofit treatments that were field 
tested under early Bureau contracts 
proved to be both effective and durable. 
The design concepts studied more recently 
for new plants provide plant designers 
with techniques to minimize unnecessary 



noise exposure and thereby reduce the 
eventual noise control costs. 

While there are some plant areas that 
can benefit from additional research, the 
techniques and materials that are cur- 
rently available make it possible to 
achieve meaningful reductions in the 
noise exposures of personnel for both new 
and existing plants. 



130 



OVERVIEW OF BUREAU OF MINES HEARING PROTECTION RESEARCH 
By Gerald W. Redmond 1 and J. Alton Burks 2 



ABSTRACT 



Hearing protective devices (HPD's) can 
be a useful adjunct in an overall program 
designed to control the noise exposure 
of miners. Performance data on HPD's ob- 
tained using standard laboratory measure- 
ments appear to overestimate the amount 
of protection from noise overexposure a 
working miner may receive from these de- 
vices. In order to more effectively 
evaluate hearing protector performance, 
the Bureau of Mines is investigating the 
basic parameters that influence the noise 
attenuating properties of HPD's. Initial 



phases of this research have focused on 
ear muff -type protectors. Alternative 
methods for the determination of total 
attenuation of ear muffs have been com- 
pared with the standard laboratory proce- 
dure and potential problem areas have 
been defined. Investigation of human 
physiological parameters that may affect 
the intrinsic noise level under a single 
earcup is being made to measure the abso- 
lute or baseline value that might be used 
as a reference level for the physical 
measurement of ear muff attenuation. 



INTRODUCTION 



Exposure to high levels of noise is 
recognized as a serious potential health 
hazard. Hearing loss due to noise over- 
exposure in industry is well documented. 
In addition to hearing loss, the litera- 
ture contains references of nonauditory 
effects of noise exposure that indicate 
general stress reactions, disturbance of 
sensory functions such as vision, and 
impairment of task performance and per- 
ception of speech. Included are reports 
of increased cardiovascular disease, gen- 
eral increases in various medical prob- 
lems and absenteeism, higher accident 
rates, etc., in noise — exposed workers 
when compared with groups of workers with 
less severe noise exposure. Consequent- 
ly, various criteria have been developed 
to eliminate or minimize the potential 
hazards of noise overexposure. 

Mining is a noisy industry. Mechanized 
operations provide the most severe noise 
exposures. In surface mining, jumbo rock 
drills typically produce noise levels in 
the range of 110 to 120 dBA, and bull- 
dozer operators are often exposed to 



Industrial hygienist. 
^Physical scientist. 
Pittsburgh Research Center, 
Mines, Pittsburgh, PA. 



Bureau of 



levels that may exceed 100 dBA. In un- 
derground mining, hand-held percussion 
drills and automated mining machines, 
such as a continuous miner, produce noise 
levels often greater than 90 dBA. It is 
not unlikely then that a significant por- 
tion of miners are chronically exposed to 
levels of noise that may be considered 
harmful to hearing. 

The Code of Federal Regulations (CFR 
30) provides regulations for the assess- 
ment and control of noise exposure to 
miners in the mineral resource industry. 
Although minor differences in the regula- 
tions exist for various types of mining, 
the primary goal in each is to prevent 
permanent hearing loss in miners. An 
exposure level of 90 dBA of continuous 
noise is allowed for an 8-h shift, with 
5-dBA increases in noise level allowed 
for a successive reduction of one half 
the allowable exposure time, up to a max- 
imum of 115 dBA for 15 min. Levels must 
be monitored to insure compliance. If 
overexposures are observed, they must be 
brought into compliance. Engineering 
and/or administrative control procedures 
are required to abate the overexposure. 

Engineering control involves the reduc- 
tion of noise by modification of the 



131 



source or by preventing noise generated 
by the source from reaching the worker. 
The noise pathway and/or the noise inten- 
sity can be reduced to acceptable levels 
in the worker's environment. This method 
is preferable since it places no burden 
on the worker to reduce the hazard. Usu- 
ally some time is required to install en- 
gineering controls on a piece of equip- 
ment, and unless the equipment is out of 
operation, overexposure can continue. 
Occasionally these controls, while reduc- 
ing the worker's exposure to noise do not 
entirely eliminate the problem and some 
degree of overexposure persists. In some 
instances, the technology to control a 
particular source of overexposure is not 
available. 

Administrative control involves re- 
structuring work patterns or product flow 
to reduce workers' risks of overexposure. 
For example, workers who are exposed to a 
high noise level may be shifted to a less 
noisy job for part of their working time 
to allow their total exposure to the 
health to be brought within acceptable 
limits. This control method is often 
difficult to implement. It can cause 
significantly higher production costs, 
require greater numbers of trained em- 
ployees for some tasks, and, if the con- 
tinuous noise level is greater than 115 
dBA, it cannot be used. 

As a temporary measure until engineer- 
ing-administrative controls are in place, 
personal HPD's may be used. CFR 30, part 

70, subpart F, paragraph 70.510 and part 

71, subpart I, paragraph 71.805 require a 
"continuing, effective hearing conserva- 
tion program to assure compliance," which 
includes the availability of personal ear 
protective devices to miners. 



program designed to protect the worker's 
hearing when additional protection is 
needed or other methods of control are 
not feasible. 

Noise, or unwanted sound, of sufficient 
intensity can cause hearing damage when 
it enters the human ear. HPD's prevent 
the transmission of sound from the en- 
vironment to the ear by providing a 
barrier that does not permit the total 
sound intensity from passing through the 
protector. 

The most common types of personal hear- 
ing protectors are earplugs and ear 
muffs. Earplugs are made of pliant mate- 
rial and are intended to fit snugly into 
the outer ear canal. Ear muffs are cir- 
cumaural devices that enclose the entire 
external ear and prevent the passage of 
sound. Individual earcups enclose each 
ear and are held in place with pressure 
provided by a headband. Soft cushions 
filled with foam or fluid are intended to 
seal the space between the head and the 
ear muff. The ear muff is formed of rig- 
id, dense material for maximum protec- 
tion. The inside of the earcup is usual- 
ly filled with open-celled foam material 
to absorb high frequency sound that may 
resonate in the cavity of the muff. 

Variants of the basic types of pro- 
tectors include cotton wool, which is 
inserted into the ear canal; headsets, 
which generally contain communication 
equipment or filtering electronics to 
allow only selected sound to pass; semi- 
inserts, which rest on and occlude the 
entrance of the ear canal; and complete 
head enclosures. 

METHODS OF EVALUATING HPD's 



USE OF HEARING PROTECTIVE DEVICES 

Personal hearing protectors represent 
the least desirable means of controlling 
a worker's noise exposure. Hearing pro- 
tectors, like respirators, etc., impose 
a burden on the worker while the source 
of the environmental hazard has not been 
eliminated or altered. However, they can 
provide a useful adjunct in an overall 



ANSI S3. 19-1974 outlines standard pro- 
cedures for evaluating the attenuation 
(noise reduction) provided by HPD's. The 
real-ear method is used to measure the 
hearing threshold of a number of human 
subjects and then the measurement is 
repeated on the same subjects while they 
are wearing the HPD's. The difference 
in decibels between the unoccluded (no 
HPD worn) threshold and the subject's 



132 



threshold while wearing an HPD is the 
protector's attenuation capability. This 
is averaged for the various subjects and 
presented for each audiometric frequency. 
This laboratory method is the standard 
method most commonly used to measure the 
effectiveness of HPD's. 

A physical method is also described in 
ANSI S3. 19-1974, which uses a dummy head 
covered with simulated "human" skin made 
of a plastic material. A measurement mi- 
crophone is embedded in one ear. Hearing 
protectors are placed on the dummy head 
or into the ear canal of the dummy head, 
and a sound field is generated around the 
head. The difference, at the respective 
frequencies, between the measured inten- 
sity at some distance outside of the 
dummy head (and outside of the HPD) and 
that measured on the inside of the dummy 
ear is the attenuation of the HPD. 

LIMITATIONS TO THE USE 
OF HEARING PROTECTORS 

No HPD is a perfect attenuator that can 
eliminate all sound transmission. The 
effectiveness of an HPD to provide its 
measured attenuation depends on factors 
related to the manner in which sound en- 
ergy is transmitted through or around the 
HPD. Figure 1 presents the pathways by 
which sound can reach the ear while a 
person is wearing hearing protectors. 
These include (1) small air leaks in 



the seal between the hearing protector 
and the head surface, (2) transmission 
through the material of the HPD, (3) vi- 
bration of the HPD in response to the ex- 
ternal sound energy impinging upon it, 
and (4) bone conduction whereby sound is 
transmitted through the skull. 

Theoretical Protection Limits 

If the entrance to the ear canal could 
be occluded with a material impervious to 
sound, sound would still reach the inner 
ear through tissue and bone conduction of 
sound through the skull. This tends to 
provide a theoretical maximum amount of 
attenuation one could expect from an HPD. 
Although this limit has not been precise- 
ly determined, figure 2 illustrates the 
observed range of bone and tissue conduc- 
tion in individuals. The value varies by 
individual and frequency, but an average 
value of 50 dB below the open air conduc- 
tion level of hearing is often used as 
the maximum sound attenuation one could 
expect from a perfect HPD. 

i 
Practical Limits of Protection 

Certain design considerations tend to 
further limit the amount of attenuation 
that can be achieved by hearing protec- 
tors. These include size of protector 
and type of material used. Earplugs must 
be small enough to fit into the ear ca- 
nal. Ear muffs must be large enough to 




Bone 
and tissue 


































Protector 
vibration 




1 


' 




i 


r 






1 




Outer ear 




Middle ear 




Inner ear 








Material 
leaks 








, 


, 
















Air 
leaks 













FIGURE 1. - Noise pathways to ear when individuals are wearing hearing protectors. 



133 



20 



m 30 



t5 

z 

UJ 

I- 

5 



40 - 



50 



60 



I ' ' ' ' I 1 ' — i i ■ • ■ 

Limitation set by ear muff vibration 



Limitation set 
by earplug vibration 




Range of bone and tissue conduction 

I i i l.ii.l I 



100 200 500 1,000 2,000 5,000 10,000 
FREQUENCY, Hz 

FIGURE 2. - Attenuation limits of hearing pro- 
tectors imposed by bone conduction and protector 
vibration. 

enclose the space around the ear, but 
must be made sufficiently small to accom- 
modate the dimensional characteristics of 
the human head and minimize irregulari- 
ties over which the acoustic seal takes 
place. 

Materials used to construct earplugs 
should be pliant enough to fit snugly 
into the ear canal and prevent leaks. 
Protector earcups on ear muffs should be 
made of rigid, dense material and the ear 
muff cushions should be made of soft, 
pliant material to provide an adequate 
seal around the ear. Also, the weight of 
the material used in making ear muffs is 
an important factor in comfort and fit. 

Figure 2 indicates limitations in at- 
tenuation due to earplug and ear muff vi- 
bration. The effectiveness of the HPD is 
significantly less at lower frequencies 
(below 1 kHz for plugs and below 500 Hz 
for muffs). In this frequency range the 
overall performance of the HPD is con- 
trolled by the transmission loss charac- 
teristics of the materials used in con- 
struction of the device. This limit, 
rather than bone conduction, determines 
the maximum attenuation one might expect 
from an HPD. 



Sound consisting of frequencies with a 
wavelength on the order of the size of 
the earcup of an ear muff can pass into 
the protector and resonate. The earcup 
acts as a Helmholtz resonator and the 
sound intensity can be amplified rather 
than reduced. Open-celled foam is gener- 
ally placed on the inside of the earcup 
to minimize this phenomenon. 

At best then, a good HPD can be ex- 
pected to provide a maximum average at- 
tenuation of 25 to 35 dB. As a rule, ear 
muffs offer greater attenuation at higher 
frequencies than earplugs (fig. 3). A 
combination of the two generally provides 
greater protection than each one individ- 
ually (fig. 4). Although the total pro- 
tection at any given frequency is not 



CO 



10 



20 



< 30 



50 



*--- 


1 ' ■ 1 ■ ' ' ■ 1 


i 
Earplug 


i ' ' 
i . . 


i 


- 


>-- 

Ear muff 

i , . i .... i 



125 



250 



500 1,000 2,000 4,000 8,000 
FREQUENCY, Hz 

FIGURE 3. - Comparison of the attenuation of 
an ear muff and an earplug. 



10 



m 20 



< 30 

z 

UJ 

£ 40 



50 



i i — I — i — i — r— r 



Earplug 




and ear muff 







_! 



100 200 500 1,000 2,000 5,000 10,000 
FREQUENCY, Hz 

FIGURE 4. - Mean attenuation of an earplug, 
an ear muff, and combination. 



134 



merely the simple sum of the individual 
attenuations, a rule of thumb estimates 
the average resultant attenuation of 
about 6 dB greater than the higher atten- 
uation of the individual HPD at most test 
frequencies. 

FIELD PERFORMANCE OF HPD's 

Laboratory measurements of real-ear 
attenuations provided by HPD's tend to 
approach those practical reductions in 
sound intensity that might be achieved. 
There is evidence to suggest that the ef- 
fectiveness of HPD's when worn under 
working conditions is not as great as 
measured in the laboratory. Thunder 
(1_), 3 in a comparison of a specific ear 
protector on "normal" and hearing im- 
paired subjects, indicated that the real- 
ear method as described in ANSI S3. 19- 
1974 may overestimate the attenuation of 
HPD's used in a noisy environment. 

In a NIOSH-sponsored study, Edwards (2- 
3^) indicated that for noise reduction by 
earplugs, noise attenuation levels mea- 
sured in the field were only 35 to 50 pet 
of the potential attenuation levels mea- 
sured in the laboratory. 



A study done by Holland (4^) compared 
the use of V51-R earplugs with attenua- 
tion of noise achieved by applying finger 
pressure on the tragi (the tragus is the 
fleshy protrusion at the front of the ex- 
ternal opening of the ear) to close off 
the ear canal; placing the fingers in the 
ear canal; and firmly pressing the palms 
of the hands over the external ear. The 
results indicated that the use of tragus 
pressure provided the most effective at- 
tenuation, palm pressure and fingers 
slightly less attenuation, and the ear- 
plug the least attenuation. 

Berger (5) compiled data shown in ta- 
ble 1 , which compares laboratory or manu- 
facturers data on the noise reduction 
rating with observed field measurement's 
of noise reduction for various HPD's. In 
all cases , the laboratory measurements 
indicate greater average attenuations 
than field measurements. These data were 
compiled from several sources and may 
reflect some variation because of mea- 
surement technique, differences in sample 
size, and other factors, but the trend is 
consistent. 

Air Leaks 



-^Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this paper. 



It is apparent that noise reductions 
achieved in the field are much lower than 
those demonstrated in the laboratory. 



TABLE 1. - Comparison of laboratory- and field-measured 
noise reduction rating (5) , decibels 





Laboratory 


Field 


No. of 
measurements 


Earplugs : 


29 
17 
15 
23 
14 
16 
26 

23 
22 
23 
23 
25 
20 


13 
6 
3 
2 
2 
3 
7 

15 
12 
11 
11 
20 
14 


152 




291 


V51-R 


95 
296 


MSA Acuf it 


13 




56 


Ear muffs: 

Safety Supply 258.... 
MSA MK IV 


18 

17 
15 
58 
47 


Welsh 4530 


5 




101 



135 



The primary cause for this is due to im- 
proper seal of the HPD, which allows air 
to leak between the HPD and skin. An 
0.5-mm air leak can reduce the attenua- 
tion of a hearing protector by about 5 to 
10 dB. Small leaks significantly reduce 
the low frequency attenuation of sound 
and as the leak becomes larger, attentua- 
tion is reduced at all frequencies (6) . 

Seal leaks are primarily caused by poor 
or improper fitting HPD's. Ear muffs 
worn over long hair, eyeglasses, or other 
objects provide less attenuation because 
of leakage caused by an incomplete seal. 
Nixon (7) reported that ear muffs worn 
over eyeglasses lose from 1 to 10 dB of 
attenuation at the individual frequen- 
cies. Ear muff cushions can become stiff 
and crack because of age, which also 
causes air leaks. Unless earplugs are 
seated properly in the ear canal, a tight 
seal does not occur between the earplug 
and the surface of the ear canal, thus 
causing air leakage and a reduction in 
attenuation. The headband on an ear muff 
or ear-canal-cap-type protector loses 
compliance after some usage. This re- 
duces the force placed upon the HPD, 
which in turn reduces the effectiveness 
of the seal and allows air leakage. When 
HPD's are worn in the field, body move- 
ment, jaw movement, etc., can create 
small air leaks in the seal. 

Thus , leakage of air around the protec- 
tor is the primary factor limiting the 
amount of noise reduction afforded by a 
HPD. Under laboratory conditions , those 
factors that allow air leakage can be 
adequately controlled. However, in the 
field the same control is not possible, 
and expected attenuations are rarely 
achieved. 

Comfort 

Comfort of fit is another factor to 
consider in evaluating an HPD. Tight- 
fitting earplugs or ear muffs can effec- 
tively reduce noise, but can become un- 
comfortable upon extended wear. Soft 
materials can be used for the plug con- 
struction or the ear seals of muffs, but 
in general these materials are not as 



effective in preventing the transmission 
of sound (material leaks). A compromise 
must usually be reached, which tends to 
reduce the potential effectiveness of the 
HPD. 

Speech Communication and Acoustic Cue s 

In many noisy environments , workers 
must be able to communicate and hear 
warning signals as well as other acoustic 
cues. HPD's can interfere with the abil- 
ity of workers to receive acoustic infor- 
mation necessary to perform their jobs 
safely. The effect on communication is 
varied. In a quiet environment, the use 
of HPD's can reduce the intelligibility 
of speech unless the speech sound is of 
sufficient intensity. At about 85 to 90 
dBA, various sources indicate that HPD's 
may improve intelligibility of speech and 
other signals. This is due to a mutual 
reduction in the signal and noise, which 
prevents overloading of the hearing mech- 
anism and allows the ear to handle the 
signal more efficiently. Kerivan (8) re- 
ports that pitch discrimination in noise 
by workers in a submarine engine room was 
reduced by the use of earplugs. Spectral 
cues necessary for excellent performance 
was impaired to a greater degree (25 pet) 
by the use of ear muffs. 

Durkin (9) found that "for speakers 
talking at normal levels of 65 dBA or 
less , . . . hearing protectors signifi- 
cantly reduced speech intelligibility." 

In an in-flight evaluation of four 
aural protectors, Parker (10) reported 
that two problems associated with the use 
of earplugs were comfort and interference 
with cabin communication. 

Russell (11) reported impairment of 
localization (that ability to identify 
direction of sound) with the use of 
HPD's. 

Howell (12) investigated the effect of 
HPD's on speech communication. It was 
concluded that "when hearing protectors 
are worn by both talker and listener, the 
composite effect is an overall reduction 
in speech intelligibility." 



136 



Saperstein (13) suggested that roof 
talk (audible signals emanating from the 
roof in mining operations) could be dis- 
criminated equally well when wearing 
hearing protection as when not if the am- 
bient noise level was sufficiently high 
to warrant the use of HPD's. 



The above results are quoted for normal 
hearing. The effect observed for hearing 
impaired is uncertain. Additionally, the 
worker may perceive problems with HPD's, 
which will often affect acceptance of 
their use. 



PREVIOUS WORK 



Early efforts in the study of noise and 
hearing protection in underground coal 
mines was done under a Bureau of Mines 
contract (14) . Part of the research in- 
volved investigation of the ability of 
miners to understand speech of cowork- 
ers and hear roof talk signals with and 
without standard ear protectors. The 
findings indicated that below 90 dBA, 
discrimination of speech and roof talk 
signals were worse when hearing protec- 
tion was provided for both a group of 
normal hearing subjects and a group of 
subjects with a simulated hearing loss. 
At 90 dBA and higher, discrimination of 
roof talk in the two groups was compar- 
able or slightly better when hearing pro- 
tection was provided. In general, the 
conclusions drawn confirmed that the use 
of ear protectors in a noise environment 
of 90 dBA and above does not additionally 
impair discrimination of speech and roof 
talk signals. The results further indi- 
cated that hearing protection should only 
be used by miners if the sound level ex- 
ceeded 90 dBA. 

A discriminating ear muff was developed 
by the Bureau (9) to allow protection 
when noise exceeded 90 dBA, while improv- 
ing discrimination of speech and warning 
signals. An electrical system was incor- 
porated into the muff so that inputs hav- 
ing a sound pressure level of 83 dBA or 
less were passed unaffected to the ears 
of the wearer. Inputs greater than 83 
dBA were progressively attenuated as the 
level increased to an upper limit of 90 
dBA when the input level was 120 dBA. In 
tests performed at the Pennsylvania State 
University, discrimination scores of sub- 
jects were significantly improved below 
and above 90 dBA as compared with results 
of tests with normal protective devices. 



Field tests noted good protection while 
providing adequate communication in low- 
noise environments. 

Research conducted by Stewart (15-16) 
studied the noise attenuating properties 
of ear muffs worn by miners. The re- 
search effort was directed towards the 
study of the attenuating properties of 
ear muffs and development of a simple 
method to determine noise reductions pro- 
vided by HPD's in the field. 

Volume 1 of the report compared the 
measured attenuation of the standard psy- 
chophysical real-ear method to attenua- 
tion measured by a physical method that 
might be suitably adapted for field mea- 
surements. In the laboratory, this type 
of study permitted the control of several 
variables that may influence attenuation 
measurements in the field. The experi- 
ment was conducted using normal hearing 
human volunteers. Real-ear attenuations 
were measured for five different ear 
muffs using 12 subjects. With the same 
subjects, a physical measurement of at- 
tenuation was performed using calibrated 
microphones placed on the outside and 
inside of the earcup and measuring the 
incident sound field and the sound trans- 
mitted through and around the muff. The 
difference represented the attenuation of 
the muff. Test results indicated that 
average ear muff attenuation measured in 
the frequency range of 125 Hz to 2 kHz 
was comparable for both methods. Above 2 
kHz, the difference in attenuation varied 
from 3 dB at the audiometric testing fre- 
quency of 3.15 kHz to about 7 dB at 6 and 
8 kHz. For all subjects and all muffs, 
the differences were consistent with the 
physical measurement method, providing 
lower values at frequencies above 2 kHz. 



137 



A reason postulated for this observation 
was that at higher frequencies, the 
shorter wavelength allowed amplification 
associated with resonance of the open ex- 
ternal ear, which is eliminated by wear- 
ing of an ear muff. 

Volume 2 of the report discusses the 
application of the information obtained 
in the first phase of the study to the 
development of a laboratory procedure to 
measure physical attenuation as a pre- 
dictor of real-ear measurements of ear 
muffs. 

Dosimeters were modified to account for 
the observed differences between the 
physical and psychophysical measures pre- 
sented in earlier work. A system with 
linear response was used to measure the 
sound level outside an ear muff worn by a 
human subject, while a system incorporat- 
ing a "correction" filter to compensate 
for the average differences in the physi- 
cal and real-ear methods was used to mea- 
sure the inside muff noise levels. 

This method was moderately successful, 
but some apparently severe limitations 
were discovered in the adaptation of the 
method in the field. With reasonable 
control over the stimulus parameters and 



other experimental parameters , it was 
possible to use the physical measurement 
procedure to predict the psychophysical 
method of evaluation in the laboratory. 
However, positioning of the instrument on 
the subject was found to be important. 
Contact with the subject's body and mi- 
crophone cable movement resulted in re- 
cording of a high noise level under the 
muff. To provide an adequate signal-to- 
noise ratio, the stimulus sound pressure 
levels needed to be at least 100 dB in 
each one-third octave band of noise. 
Average attenuation brought the level of 
sound in the higher frequencies (2 to 8 
kHz) inside the muff close to 65 dB. The 
measured noise floor under the muff was 
57 dB. This difference in signal detec- 
tion is marginal. In the field, it is 
not common that sound pressure levels in 
the third octaves in the region above 2 
kHz would exceed 100 dB. This would tend 
to invalidate the results. 

At lower frequencies where ear muff at- 
tenuation is much less, the needed sig- 
nal level for adequate high frequency re- 
sponse resulted in instrument overload. 

The conclusion was that these consider- 
ations would severely limit the applica- 
tion of the method to field use. 



CURRENT AND FUTURE BUREAU OF MINES INVESTIGATIONS OF HPD's 



With emphasis placed upon the use of 
HPD's as an adjunct to an adequate hear- 
ing conservation program in industry, it 
has become increasingly important to de- 
velop a simplified method of evaluating 
the performance of HPD's worn by miners 
in the field. 

The immediate objective of the Bureau 
study of personal HPD's is to investigate 
their attenuation characterisitics. Per- 
formance data obtained using a standard 
psychophysical method in the laboratory 
appears to overestimate the amount of 
protection against noise overexposure 
provided by HPD's when used by the worker 
in the field. In order to more accurate- 
ly measure the actual reduction of noise 
afforded the working miner by HPD's, 
basic parameters effecting the acoustic 



performance of HPD's are being investi- 
gated. Laboratory methods providing sig- 
nificant control of these parameters are 
being developed, which predict the at- 
tenuation of hearing protectors using 
standard methods of measurement. The ul- 
timate goal of the study will be to de- 
velop a laboratory procedure that can be 
adapted to measure the degree of protec- 
tion provided by HPD's in the field. 

The current study of hearing protec- 
tor performance characteristics will ini- 
tially evaluate work performed under 
Bureau contract JO188018, "Noise Attenu- 
ating Properties of Ear Muffs Worn by 
Miners." Limitations to the development 
of the method proposed under that study 
will be looked at more intensively. 
Alternative measurement techniques and 



138 



state-of-the-art signal and correlation 
analysis will be used to investigate the 
various parameters of limitations such as 
the measured noise floor under the muff. 
The causes of those limitations will be 
identified and assessment made of the de- 
gree to which those parameters affect the 
measurements and to what degree they can 
be controlled. 



1. Identification of the parameters 
affecting the noise attenuation charac- 
teristics of HPD's. 

2. Development of measurement methods 
to evaluate the parameters involved in 
hearing protector performance. 

3. Evaluation of equipment needs. 



Alternative methods for determining 
attenuations in the field need to be pos- 
tulated and investigated. The procedure 
outlined by Stewart (16) is specific for 
ear muffs. It is desirable to devise a 
testing procedure that is generally 
applicable to all types of HPD's. Ear- 
plugs would be difficult to evaluate 
using Stewart's method. Therefore, seri- 
ous thought must be given to the appli- 
cation of this method and others to 
earplugs. 

The research can be expanded to provide 
a laboratory method of evaluation for 
HPD's that can accurately predict attenu- 
ations observed by the standard real-ear 
method. This has already been demon- 
strated for ear muffs. 

Upon development of a field method of 
evaluation of the attenuation character- 
istics of HPD's, it will be necessary to 
demonstrate the procedure in the field. 
Acquisition of field data is therefore 
anticipated as a future effort. 



4. Assessment of the effect of the 
measurement method in altering the atten- 
uation characteristics of the hearing 
protector. 

5. Quantification of the degree of 
protection from hearing loss afforded by 
the protective device. 

The research plan includes the follow- 
ing tasks: 

1. Evaluate the limitations expressed 
in reference 16 — 

a. Quantify the noise floor gener- 
rated under an ear muff when worn. 

b. Evaluate if possible the physi- 
cal and physiological components of the 
noise floor. 

c. Determine what degree of con- 
trol can reasonably be anticipated over 
those parameters in attempting to lower 
that noise floor. 



The information gained in the program 
may additionally be used to provide de- 
sign criteria for hearing protectors used 
by miners that will provide adequate pro- 
tection under noisy conditions while not 
reducing miner safety. 

Status of Current Work 

The ultimate objective of the program 
is to develop a method for evaluat- 
ing hearing protector performance that 
might be adapted for use in the field. 
The initial phases of such a task is 
much more fundamental. Considerations 
currently being addressed include the 
following: 



d. Assess and attempt to minimize 
instrument effects in the measurement of 
attenuation. 

e. Evaluate level and spectral at- 
tributes necessary for making valid mea- 
surements that can be related to the 
standard psychophysical method of attenu- 
ation measurement. 

2. Propose and investigate a method 
that is generally applicable to all types 
of HPD's. 

3. Postulate and evaluate alternative 
methods of measurement of attenuation. 



139 



4. Develop a laboratory measurement 
procedure based on prior investigations 
of the parameters of measurement. 

5. Demonstrate the measurement tech- 
nique under simulated conditions typical 
of work situations. 

6. Field evaluate the method. 

To date, a literature review has been 
performed and pertinent articles se- 
lected. This task is continuing and will 
be updated as cogent information develops 
in the literature. 

Investigations have focused on the 
study of ear muff attenuation to main- 
tain continuity with previous contract 
work and because methods developed under 
that contract are more applicable to ear 
muffs. The most severe restriction im- 
posed by the research plan was the lack 
of a sufficient signal-to-noise ratio to 
make a suitable measurement of the atten- 
uation of an ear muff at high frequen- 
cies. This was attributed to a relative- 
ly high noise floor under the earcup of 
the muff when placed on the human head 
and sealed as tightly as possible. 

The first series of experiments was de- 
signed to confirm and quantify the pres- 
ence of a noise floor under an ear muff 
worn by a human subject. The experiments 
were conducted in a large anechoic cham- 
ber where the performance of hearing pro- 
tectors on human subjects could be accu- 
rately measured. Although the subject 
remained passive in the experiment (no 
human response was measured or necessary 
except for cooperation in remaining as 
quiet as possible during measurement pe- 
riods of the various levels at the fre- 
quencies of interest) , it was felt that 
certain characteristics of the human ear 
such as impedance, etc., could not easily 
or accurately be accounted for by use of 
things such as a dummy head. By using 
human ears these considerations could be 
neglected. 

A Knowles BT 4 series 1759 microphone 
was selected for measurement of the noise 



floor under the muff. This subminiature 
electret condenser microphone was se- 
lected because it had a reasonably flat 
frequency response in the frequency re- 
gion of interest for measurement (31.5 Hz 
to 8 kHz). Its dimensions were small so 
that it would not appreciably affect the 
volume under the earcup and its weight 
(0.28 g) would not appreciably add to the 
mass of the earcup. It could easily be 
attached to the entrance of the ear canal 
of the subject to measure the noise floor 
at that point. The microphone was cali- 
brated at 1 kHz and several other fre- 
quencies using an insert voltage tech- 
nique. A standard Bruel and Kjaer (B&K) 
type 4160 condenser microphone was used 
as reference. 

Initially, one subject was tested using 
one ear muff, the MSA Mark V. In order 
to evaluate the effect of some physio- 
gnomic and physical parameters that might 
influence the measurements , nine other 
volunteer subjects were recruited. These 
included three females , three bearded 
males , and three clean-shaven males . The 
Knowles microphone was taped to the sub- 
ject's neck just under the ear lobe with 
the microphone placed just outside the 
entrance of the ear canal and the leads 
firmly attached to the recessed area be- 
tween the jaw and the back of the neck. 
This arrangement allowed a firm seal of 
the ear muff over the lower portion of 
the ear. The right ear was used for all 
subjects. Coaxial wires were then at- 
tached to a 1617 B&K band pass filter and 
a 2606 B&K measuring amplifier outside of 
the anechoic chamber. The output of the 
measuring amplifier was then fed into a 
B&K model 2305 strip-chart recorder. 

The environmental levels of the noise 
in the chamber with the subject present 
were first determined. The muffs were 
then fitted tightly onto the subjects, 
and the noise floor determined at the 
frequencies of 31.5 Hz, 63 Hz, 125 Hz, 

4 Use of manufacturer or brand names is 
for identification purposes only and does 
not imply endorsement by the Bureau of 
Mines. 



140 



250 Hz, 500 Hz, 1 kHz, 2 KHz, 4 kHz, and 
8 kHz, as before. The average measured 
noise floor ranged from 57.7 dB at 31.5 
Hz dropping to 16.0 dB at 500 Hz and ris- 
ing slightly to 21.7 dB again at 8 kHz. 
The respective average environmental lev- 
els at these frequencies were 29.7, 16.1, 
and 21.7 dB. 

The same measurements were made with 
the subjects wearing glasses or by simu- 
lating a small air leak near the temple. 
Average values for the noise floor at 
31.5 Hz, 500 Hz, and 8 kHz were 46.2, 
16.0, and 21.7 dB. 

The data are currently being analyzed 
and the human physiological parameters 
that influence' the measurements are being 
identified. Preliminary analyses tend to 
attribute the presence of the noise floor 
to human respiration and pulse, but the 
magnitude and frequency characteriza- 
tion of these two parameters have not yet 



been determined. These factors appear to 
have a significant influence on the noise 
floor under the muff only at frequencies 
less than 500 Hz. The problems antici- 
pated in the investigation implied diffi- 
culties with inadequate signal-to-noise 
ratios at the higher frequencies for am- 
ple measurement. The procedure developed 
for the physical measurement of attenua- 
tion showed close agreement with the 
standard psychophysical measurement at 
frequencies below 500 Hz. 

Information from these initial studies 
require further analysis. Other experi- 
ments are being designed and conducted to 
further clarify the relationships and 
significance of the various parameters 
influencing the measurement of the at- 
tenuation of HPD's. These measurements 
are prudent and necessary for establish- 
ing baseline data for future investiga- 
tions into the performance of hearing 
protectors. 



REFERENCES 



1. Thunder, T. D., and J. E. Lankford. 
Relative Ear Protector Performance in 
High Versus Low Sound Levels, J., Am. 
Ind. Hyg. Assoc, v. 40, No. 12, Dec. 
1979. ' 

2. Edwards, R. G. , W. P. Hauser, N. A. 
Moiseev, A. B. Broderson, W. W. Green, 
and B. L. Lemper t. A Field Investigation 
of Noise Reduction Afforded by Insert- 
Type Hearing Protectors. HEW (NIOSH) 
Publ. 79-115, 1978, 43 pp. 

3. Edwards, R. G., W. P. Hauser, N. A. 
Moiseev, A. B. Broderson, and W. W. 
Green. Effectiveness of Earplugs as Worn 
in the Workplace. Sound Vib. , v. 12, No. 
1, Jan. 1978, pp. 12-10, 22. 

4. Holland, H. H. , Jr. Attenuation 
Provided by Fingers, Palms, Tragi, and 
V51-R Ear Plugs. J. Acoust. Soc. Ameri- 
ca, v. 41, No. 6, June 1967, 15-45 pp. 

5. Berger, E. H. Comparison of Labo- 
ratory Measured Noise Reduction Ratings , 



(NRR) With Those Measured in the Field. 
Pres. at OSHA Hearings on Noise in the 
Work Place, Dep. Labor Docket 329-37C, 
Standard 1910-95, Noise. 

6. Olishifski, J. B., and E. R. Har- 
ford. Industrial Noise and Hearing Con- 
servation. National Safety Council (Chi- 
cago, IL), 1975, 1120 pp. 

7. Nison, C. W. , and W. C. Knoblach. 
Hearing Protection of Earmuffs Worn Over 
Eyeglasses. Aerospace Med. Res. Lab. 
(Wright-Patterson AFB, OH) Rep. AMRL-TR- 
74-61, June 1974, 31 pp. 

8. Kerivan, J. E. The Effects of Ear- 
plugs and Earmuffs on Pitch Discrimina- 
tion in Noise. Naval Submarine Res. Lab. 
(Groton, CT) Interim Rep. NSMRL-888, Feb. 
2, 1979, 15 pp. 

9. Durkin, J. Effect of Electronic 
Hearing Protectors on Speech Intelligi- 
bility. BuMines RI 8358, 1979, 19 pp. 



141 



10. Parker, J. F., Jr. Protection 
Against Hearing Loss in General Aviation 
Operations, Phase 2. Final Rept. , con- 
tract NASW-2265, Sept. 1972. 

11. Russel, G. , and W. Noble. Local- 
ization Response Certainty in Normal and 
Disrupted Listening Conditions; Toward a 
New Theory of Localization. J. Auditory 
Res., v. 16, No. 3, July 1976, pp. 143- 
150. 

12. Howell, K. , and A. M. Martin. In- 
vestigation of the Effects of Hearing 
Protectors on Vocal Communication in 
Noise. J. Sound Vib. , v. 41, No. 2, July 
1975, pp. 181-196. 

13. Saperstein, L. W. , and W. W. Kauf- 
man. Audible Warning Signals in Under- 
ground Coal Mines. Trans. Soc. Min. Eng. 
AIME, v. 258, No. 1, Mar. 1975, pp. 1-7. 



14. Pennsylvania State University. 
Aspects of Noise Generation and Hearing 
Protection in Underground Coal Mines 
(grant GO122004). BuMines OFR 19-73, 
1972, 158 pp. 

15. Stewart, K. C, and E. J. Burgi. 
Noise Attenuating Properties of Earmuffs 
Worn by Miners. Volume 1: Comparison of 
Earmuff Attenuation as Measured by Psy- 
chophysical and Physical Methods (con- 
tract J0188018, Univ. PA). BuMines OFR 
152(l)-83, 1980, 46 pp.; NTIS PB 83- 
257063. 

16. . Noise Attenuating Proper- 
ties of Earmuffs Worn by Miners. Volume 
2: Development of a Laboratory Procedure 
for the Physical Measurement of Earmuff 
Attenuation (contract JO188018, Univ. 
PA). BuMines OFR 152(2)-83, 1980, 37 
pp.; NTIS PB 83-257071. 



*U.S. GPO: 1984-705-020/5034 



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